Good point about knowing the technical limitations.
12 colors for an individual character sprite. If the sprite is armed for battle, that limiit goes up to 15 if I'm not mistaken (12 for the sprite, 3 for the weapon). Also, if the sprite is a player character, each sprite frame must fit into four tiles' worth of space. A "tile" is composed of four 8x8 pixel squares. Here's a visual of a tile:
Player characters are all "Size 0" sprites currently, and are composed of four tiles. Size 1 sprites are composed of 8 tiles. Size 2 sprites are 12 tiles. If the sprite is an NPC or enemy, it should be safe if it consists of up to twelve tiles. Vehek determined that Size 2 sprites were the maximum used in Chrono Trigger. A neat trick for huge bosses is making much of them part of the background.
It's my sincere hope that we'll figure out the memory management issue preventing Size 1 sprites for player characters one day.
But once the spriter has a handle on those few facts, it's pretty cut-and-dry: we need artistic skill. The technical side can be learned with a bit of patience, and I solemnly stand ready to insert any sprite sheet a spriter could devise.
EDIT: Post corrected per Vehek's observation.