To anwser the original question I would say the fleets get as big as the RT in question both want and are able to make them, HOWEVER a large RT fleet is not realisitcally going to be in one place all at once. Most of the other ships are going to be charter vessels making milk runs or captained by trusted vassels and bannermen off protecting various interests, conducting business and working on approved side projects.
My players are getting really expansive and ambitious about their fleet and I've basically told them that's fine but very few tasks are going to justify the deployment of more than one to three of their favorites and subsequently rolling around with an entourage of ten ships is going to be anti-profitable for most tasks.