After looking at this picture, I started thinking, maybe we should have a 'feminine penis' tag. What do you think?
I mean, you can see that his (or her) penis (PENIS) is pretty small compared to the masculine guy's large man-penis (man penis (PENIS)) which is my reason for thinking that. But I guess a 'small penis' tag would be less specific and perhaps better.
Little penis is a tag. Besides that, "micropenis" is the medical term (and is part of the "intersex" spectrum).