The way the buildings are stable is that you have to put pillars. But yeah there should be a limit where if it is too high, you have to expand your bottom floor up (like if you reach that limit, you have to expand the base of the raft until its 5x5.... its kind of confusing to say :P)
the floor above should have to be 5/4 x as big as the one on top or something... or just add weight so if its to big it starts falling apart. also whats with how well these things float 0-o tale ones should sink down more.
If it's too tall, getting down to kill the shark will take longer and eventually it will collapse if you don't constantly repair it, seems like that should be enough to prevent issues.