Don’t you think only the bigger guys would sign on to donate their organs? The guys with a micro penis probably wouldn’t wish it on the recipient. “Here, it never did much for me, you try.”
I don’t recall any specifying whether I wanted to donate my penis or not, so I suspect male organ and tissue donors in general are the candidates, with blood type and possibly skin tone being considerations. If I were one of these poor guys I’d be awfully disappointed if my “new one” was less than impressive, but I think I’d get over it. Better than nothing at all.