Well, I think this is two separate questions. First, I don’t think we need to worry about any invasion, for the simple reason that if any alien wanted Earth they wouldn’t wait for us to evolve but would have taken it over millions of years ago. The solar system and entire galaxy would be full of aliens if they were an expansionist colonizing species. That’s because the chance of an alien evolving to technology at the exact same moment of time as us is tiny, over the billions of years since life first became possible in our galaxy, since at least double the age of our sun.

If there are aliens here, then they obviously are not the colonizing type. And I think there’s a reason for that - that any species that set out to colonize the galaxy would be a major risk to itself, filling the galaxy with evolving creatures and their biological and machine creations. It would be far worse than filling the galaxy with self replicating machines, because any of their descendants could do that, but they could do many other things also, and the most aggressive expansionist ones with the fastest increasing population would win the race to fill the galaxy. I think any ET would look at that prospect with dismay and figure out a way to avoid it. Because with even slow exponential growth, say a century before you replicate, you could fill the galaxy with your species, crowding every star system to the max, within a million years, at an average speed of a tenth of the speed of light at an average speed of a tenth of the speed of light.

So all those alien invasion scenarios thought a lot of fun I think are just not credible. Far more likely to be visited by robots,as that’s a safe way to explore a galaxy, if you take a lot of care in their construction and design, and they could be designed for safe self replication, which biological and evolving creatures can not be. Or if they are extra terrestrials, then somehow they have found a way through this.

Also - it’s extremely unlikely that we encounter extra terrestrials during a phase of rapid exponential growth, doubling every century or even less, because that just can’t last for more than a short period of time. I figured out that if we doubled in population every century, and the same size as the smallest humans, then by 13,000 years, we need to convert an entire galaxy into humans every 100 years. And within 18,000 years we need all the matter in the observable universe every 100 years. Just to make humans, never mind our life support. Make the doubling time 1000 years instead of 100 and the limit is still only 180,000 years before you need to convert the entire visible universe into humans every century to keep going.

For more on this.

Why ET Populations Can't Continue To Expand For More Than A Few Millennia
Why Only Very Young ET Civilizations Will Have Expanding Populations - Opinion Piece

So, I think there is almost zero chance of encountering extra terrestrials in a phase of exponential growth. That can last at most a few thousand years and certainly less than a million years unless they have a very very long lifetime. It is possible that we find exponentially growing ET populations that double only once every few million years because they have very long lifetimes perhaps and only reach maturity at age a few million. I don’t know how likely that is, just mentioning it as a mathematical possibility.

Apart from that, I expect them to have stable populations and not to be expanding when we contact them. And because our galaxy is not filled already with warring extra terrestrials, their population is likely to fill only part of it. Most likely just a few star systems I would guess, because otherwise - if there was a sub population that started to grow exponentially, it would be favoured over all the others and soon fill the galaxy. To avoid that happening, the region they occupy has to be small enough so they remain in communication with each other, to avoid exponential growth, probably due to choice, that to expand exponentially is unthinkable because of the consequences of doing so.

They could fill the entire galaxy if they had faster than light travel, or again, extremely long lifetimes and reach maturity after only millions of years. That way they could traverse the galaxy several times in a lifetime, and maintain a galaxy wide civilization with shared values preventing exponential growth.

But I think the most likely scenario myself is that we encounter ETs that are based around a single star system or a few star systems and have reached a stable population and don’t expand any further. And they may well travel throughout the galaxy, but only as explorers, not as colonizers. They may also send replicating robots ,or lightweight easy to manufacture robots throughout the galaxy. They could have robotic sentinels around every star in the galaxy if they wanted to.

I think that pretty much follows mathematically (as a probability argument of course) from the assumption that a technological intelligent species has evolved in our galaxy already (if they have), and that they aren’t here already.

Now if they made them invasions by future descendants and creations of humans, which came back to invade Earth after evolving a few hundred light years away for a few centuries, that would be plausible. Or even faster if they use genetic engineering, or cybernetics, the result could be some creature as different from us as the extraterrrestrials of movie fiction within a short period of time.

I go into this in more detail in my Case For Moon First - section on habitable exoplanets

So on the second part of your question - well first there’s no limit on size in the oceans, except limits on amount of food available, which is why the blue whales can get so large. If they had plenty of food available they could get even larger. Entertaining discussion here on stackexchange: Is there a maximum size an ocean bound creature could grow to?

And on land - well it depends on what you count as a single organsims. Perhaps they swarm, like locusts, and the swarm acquires a group intelligence, which the ETs think of as themselves? They spend most of their time “asleep” with their component parts as grasshoppers but from time to time they swarm and wake up as intelligent creatures. Or they could be thin biofilms that spread over square kilometers and slowly creep around. Or they could be gas filled creatures that fill their bodies with hydrogen and float in the skies.

For land animals of a more conventional type. Mike Taylor of the university of Helsinki has done a calculation based on physical principles and came to the conclusion that land animals should be able to reach a mass of between 100 and a thousand metric tons. So why haven’t they got that large? The largest dinosaurs, Brachiosaurus, only reached 80 metric tons. The Size of the Largest Land Animal. He thinks it is because of the problem of getting enough food to support such a large mass.

Meanwhile, larger animals need a larger area to live in at least if they need to east other animals or plants to survive - animals on islands are smaller, so if you get low populations, or larger areas to live on, there’s more potential for larger animals. Land Size Limits Body Size of Biggest Animals

One scientist, Simpson, has taken this further and tried to work out the average size of an extra terrestrial.

His basic argument is that smaller aliens will have larger populations. So if you assume that we are selected at random from all extra terrestrials, we are likely to be both smaller than average and also more numerous, because amongst all extra terrestrials there would be a few planets with billions of them, and many planets with only a few millions or less. It went through peer review and was published. He estimates that the average size of an alien, by species, would be the size of a polar bear, with much smaller populations than us. So if we make contact with extra terrestrials he’d expect large ETs with a population of millions rather than small ones like us with a population of billions at least originally on their home planet. He also works out that their planet is likely to be at most 20% larger than ours. It’s extrapolating a lot from a single sample, you can see the arguments here: The Big Alien Theory

I find those arguments not that compelling myself. After all I could use a similar argument to prove that I’m probably either from India or China as the two most populated countries in the world. But in fact I’m from the UK with a comparatively tiny population. There’s a less than 1% chance of being from the UK. And even less so of living in Scotland, which is where I live, and I live on a tiny island too, and when you get down to those levels, there are 2,667 on this island, so the chance of living on this island is only 0.00004%. Wherever you live you can do a similar calculation to figure out that the chance of living there is absolutely minute, so how can you exist :).

So why couldn’t we be a rare form of ET too? But it’s a fun idea and if we encounter just a few extra terrestrial intelligent species, we may be able to answer questions like that with actual concrete numbers. Will be interesting to see if they are huge like polar bears, or tiny like parrots. Grey parrots are nearly as intelligent as humans - amongst the most intelligent of all species studied, which shows that you don’t need a big brain or body to be intelligent. I don’t see why a parrot couldn’t evolve to be as intelligent as humans. So ETs could be at least as small as grey parrots.

So anyway - if Earth like creatures, well land animals may have an upper limit but it’s quite large, perhaps between a hundred and a thousand tons. No limit for sea dwelling creatures or various types of land based or airborne creatures with different biological organization from typical Earth creatures as the locust swarm idea shows.

And if evolved around another planet - and then moving around on Earth with some technology to make it possible to move in our gravity, well again no limit to their size, seems to me,