Generally the fleet size is left at normal.

People prefer large fleet size in the early game (since it means you don't have to enter upkeep quite so early) and they prefer small fleet size in the late game (reduces lag).  So normal fleet size is actually a very nice compromise.  On small maps which seldom reach late game, large fleet size is usually favoured, while on huge maps where massive fleets are to be presumed, small fleet size is more common.

When choosing large fleet size, it's important to think carefully about the game you're playing.  Large might not be a good idea on a large map with 8+ players because it could lead to lag.  Another factor is the role of starbases.  Since a starbase's power is independent of fleet supply, they are relatively more powerful in small fleet games and less powerful in large fleet games.

In practice, this is not a limitation at all, since I've never come close to using all 16 in multiplayer.  The most I've ever used at once is about 10.  The limiting factor on capital ships is actually the amount of XP you can get for them, which does increase with fleet size, therefor capital ships actually do scale here.

Thanks everyone.  So basically, larger maps = smaller fleet size. 

I cant imagine using all 16 in Multi.  However, once a friend and i were 2v2v2 against 4 A.I.  I was testing the position of Eco., and at the end when i had 400 000 + credits in the bank with everything researched, i decided "Well, since we've already won..." scrapped 4 trade ports for 4 Cap ship factories, and built 16 Marzas (silly Akkan was destroyed due to negligence on my part a while back...)

Yeah... 16 Marzas using raze planet was... epic...