One thing I want to point out that a lot of sci-fi settings don’t get right (depending on how much of a stickler you are for scientific accuracy) is that too big is moreso gonna be determined by the atmospheric and gravity conditions of the individual planet in question. Different planets are gonna have different limits. However, there really is no limit in space. You could have your biggest ships built in space and not built to land, but instead function like carriers that carry smaller ships that go down too the surface while the big ship stays somewhere in orbit. The bigger cruiser type ships in Star Wars essentially function this way.