I definitely prefer larger playable casts in my JRPGs (or any RPGs really). Although, I stress larger, not necessarily large. 9~12 seems about right, as long as each character is unique in some way, gameplay or story-wise.

The main thing I don't like about smaller casts is that it limits character personality archetypes - honorable hero, ladies man, idealistic playful girl, tough hardened girl, crazy dude, sulker, the self-doubter, etc etc. In FF6's case, the variety of characters' personalities is what I like most. Granted, I could see the attraction to a smaller cast as it could streamline the story a bit better?