Penis proportions.

In my experience, guys almost always have penises that fit them. It's not that I can look at a guy and guess what his penis looks like, but once I see it, I can go "oh, that makes sense." Just as most people tend to have the right nose for their face, most guys have the right cock for their body.

Then again, the older I get, the less I care about anatomy at all. It's nice when it works well for me, certainly I do enjoy the PIV just a tiny bit, but a guy's ability to whisper really horrible things in my ear or find just that spot inside me matters so much more. As so often happens in my life, I knew the cliche but it took me years to really get it: it's the motion of the ocean, baby.

Even to the extent anatomy matters, I find that sheer bigness isn't it; I used to sleep with a guy of whom I always thought "gee, he has a really comfortable cock," and if this sounds like sort of a backhanded compliment, trust me, there are entire non-stop weekends that say it's not.