During the hey day of K/S fan fiction Spock’s penis came in a huge array of sizes and shapes. Gayle F was the Queen of The Bells - she had Spock with a penis that came in the shape of five bells, and that will always haunt me. And yet… ribbed… for Kirk’s pleasure. Hmm…
The classic fannon that was thoroughly adopted and turned up everywhere was the ‘double ridged’ penis. That is, Spock’s glans had a, er, kind of double echo thing, another set of ridges just underneath. A subtle nod to the alien penis and the fans’ love affair with same.
I’m pretty sure there were a load of self-lubricating penises in the stories - because you could so totally get away with that in fandom. And why not? A self-lubricating penis is logical, dammit, Jim!
There was one story (sadly, I never found this one, only heard about it in horrified whispers from friends) where Kirk had been raped and had developed a fear of penises (other people’s, one assumes, not his own), but that was just hunky dory as Spock didn’t have one. He, instead, had a hundred tiny tentacles that all sort of came together to do the job. How perfect is that? I wish I could find that story, because it’s just like mini-hentai! Hundreds of tiny tentacles! Adorable! Although I think it would make giving Spock head an adventure in eating spaghetti. “Hey, Kirk, you want parmesan on that?”
Oh, and there was this really great novel, I forget what it was called, but it was huge, where Spock’s nuts were on the inside. In fact, he may have had a couple of sets… no, wait, I remember, his nuts where accessible from the back, like, um, near his kidneys or something, so he really, really enjoyed massages, and then his scrotum just filled up with juice prior to coming. Or something. Anyway, it was hot. Back nuts.
My absolute favourite, though, was the extreme alien penis presented by Leslie Fish (Ah, Leslie, your zines are still my sugar bunny comfort fic). Spock’s genitals could best be described as a kind of hairy orchid. When he became aroused, the petals unfolded, revealing a studded (with emeralds) green shaft (again, ribbed for Kirk’s pleasure!), and two little whippy tentacles that just joyfully joined in the fun. [11]

by iibnf from Different Shapes and Sizes under Vulcan Genitalia (2007)