As a guy, I'll take a crack at this...
Make sure it can stretch as needed without injury, but can also grip, push, and pull.
Also do something to reduce/eliminate UTI's, that sucks for everyone.
There isn't so much an ideal size that everyone should aspire to, rather you should find someone that fits you. It's best when the sheath fits the sword.
Also a guy and never really thought about it. It's not like I ever had multiple options to choose between. Just have to take whatever comes with the few women that have been interested in fucking me.
This got me weirdly thinking about what would be the perfect size and whether they could genetically select for that, then I realized I was thinking way too much about a web comic and should just go see if there is a porn version of Gattaca.
Penis size is specifically mentioned in Gattaca by the character supervising the urine sample, so I'm pretty sure they're already engineering that in the world of the story