How big can a 13 year old's muscles get?
As big as you want, I wouldn't try for very big. Too much would look odd.
Why does it matter? You can get it big, but don't over do it or you will get injured. Why? Trying to pick up a chick? Tsk tsk tsk. If not, then the answer also was in this post.
You're back... Just go to a gym...
Not big. Your 13.