Luffy is the biggest (partially bc rubber) I think Zoro is girthy and sanji is long so idk how to classify is bigger like sanji I guess for length? For the second part of your question Zoro just has a big useless shlong Godspeed sanji prob loves a good cockring/cage and Luffy prob squeezes it like a stress ball, Sanji technically uses his better bc he’s more skilled and precise like he knows just where and when to hit to drive zoro over the edge/insane, Luffy is a messy fucker and just rams in however many times as he likes and def overstimulates tf out of zoro