Let's say that I had some sort of habitat in a space station or similar structure, which had everything a blue whale would need to survive, but did not have any gravity. How large could a blue whale reach in a zero gravity environment? If allowed to breed and evolve, would they become larger? (The project I am working on isn't actually with blue whales, but I felt like would be a good example of the general concept I'm thinking of, which I could use to make my question more specific) Edit: This question is ONLY using blue whales as an example of "really large animal that could function in an environment with no ground to walk on", and I am only asking about how large they could get without the restriction of gravity, not whether or not they could survive. I do not agree with the sentiment that the ocean is "already pretty much zero gravity", as there is very clearly gravity in the ocean, and even just a little bit of gravity would still have at least some sort of effect on how large an organism could grow. Even if the increase is only a tiny fraction of a percent, I still want to know that.
The main limitation on size is gonna be blood flow, and if you don't need to do much energetic stuff, it's not gonna be that hard to evolve some arteries to contract to push blood around.
The main limitation on whale size is food, and if you give them more food if they're larger you could push them to be even more massive.
You'd need to push them.
Blue whales are huge because competition is intense. They use lunge feeding, where they lunge after krill or similar prey to get huge amounts of food. This means they need huge amounts of energy for quick bursts and to have a larger mouth to catch more krill.
There are many more maneuverable or agile predators, but none with as much endurance and size of mouth as blue whales.
If they don't need to be larger to be fed, why would they evolve to be larger?
You are going to have very strange evolutionary forces if a single species and all it's ancestors never have to lift a fin for food or breeding. Perhaps it just evolves to be as cute as it can so the handlers will give them more food or attention. Sure it could get huge, but I don't think it would be a option all roads must lead to.