My preference is for a party of around six, but as not everyone can usually make it I like to have a group of about eight players.
Currently, my 'player pool' is somewhere in the 9-10 range, and a typical game has 6-8 pcs at a time. At least one of the players was kind of an 'accidental addition' when the game was already full, but damn, he brings a lot to the game (in terms of roleplaying, rules knowledge and general good behavior).
Large parties tend to slow the game down somewhat, especially if it's roleplaying heavy. Also, each character gets less spotlight time.
One of the big advantages of a large party is that the game can go on even if you're missing half the party. Now, this is totally a matter of individual gaming styles- some groups just don't like to play unless everyone's there, and some groups play no matter what; but I like to have a quorum present, which I typically set at 3 pcs. (Though this can vary.) This is easy if it's 3 out of 8. This is a great solution for the 'flaky player' syndrome that I'm sure many of us have seen- the guy who shows up late, only half the time, etc. for no reason: you just go on without him.