If you want your ship to easily travel between the Atlantic and Pacific then it has to fit through the Panama canal. This limits it to 950 feet long, 106 feet wide, and a maximum draft of 39.5 feet, and it can’t be more than 190 feet from the waterline to the highest point on the ship (numbers according to Wikipedia). This is the limit for a lot of ships these days.
If you just want something that floats, I don’t see where there’s really much of a limit at all theoretically. Just imagine bolting aircraft carriers together. There’s no reason you can’t make larger and larger groups of huge aircraft carriers all attached to each other, other than practical reasons like cost, being able to fit into harbors to dock, fuel costs, crew logistics, and that sort of thing.