Depends on your preference. Big will be good enough for a handfull to twist around. Small would be too small for a handfull, but good for a lick and suck. I'm sure you don't want to suck on a huge nipple and getting it around in your mouth like an attached grape that just won't let go? Then again, they make great locations for nipple rings!
P/S: You could always discuss male and female nipples, it makes a better topic!