Some guys like them small and some like them large. Small fists allow for easier entry, depth play, and of course double fist play. Big fists make entry, depth, and double fist play a bit more difficult, but can provide some intense ass stretching play.
For me, I like a nice big fist to really stretch and challenge my hole. I am lucky to have a top with nice big fists. Nothing makes me hornier that him getting both fists greased up and working my ass until it is wrecked.
What is your preference for fist size?