My experience is that medium sized works best for me. The whole idea is to find someone whose insertion can stimulate your prostate ( the key to being a bottom). The medium sized ones seem to be able to best hit the prostate exactly where they drive me crazy. Of course, the whole thing is dependent on the skill of the guy who is attached to the dick. I've had very large ones and very small ones ( which have a tendency to poke at the prostate, making it uncomfortable), which have been satisfying, but a medium sized one on a skilled cocksman makes me scream into the pillow and thrust my hips wildly, especially if he is mounting me from behind.