Umm Like what is the average size for a 13 year old male?
I need to know.
Not to be rude or anything; but with all the posts on this sort of topic I think we all would know by now that average size for guys that most likely haven't completely gone through puberty yet means nothing at all. There's really no answer to your question. I mean, the only way to know if a guy is normal or whatever is just to wait and see what he's like when he's fully physically matured. Sorry I couldn't be more helpful, but that's pretty much the way things are IMHO.
According to http://www.afraidtoask.com/members/mgentable.html, 3.5" (erect). Now that data is 60 years old, and people may be developing sooner now, but the average wouldn't be more than 4". That average, however, covers a huge range, from people who haven't stated puberty yet, to people who have practically finished. Anywhere along that is "normal" for being 13.