For RPGs 3-5 doesn't make a big difference to me, provided they are properly set up. If each character has to be more narrowly defined then you need more to fill all the roles in combat. If each character is more versatile then you need less.

For SRPGs I strongly feel 10 is to many. (and also apparently the standard). With a party size like that your characters movement range is so limited you spend half of the level gridlocked by your own party. Just moving across the map is a chore. It also promotes lack of diversity in party composition. If I have 10 guys I know I am going to have to have 2 mages, 2 archers, 1-2 healers, 2 heavy melee, 2 light melee. (or close to). With less party members there is usually considerably more ability to mix them up, as it goes hand in hand with giving them more versatility.