If you're going to get taller throughout this year, then get the medium, but if you are done growing(i'm assuming you're not) then get the small. But you should just get the medium because it won't get small for on you for a few years. Also if you wan't it to stay as big as possible, then don't wash it, and I know that can sound nasty but if you just hang it up every time after you wear it, then it should be fine. Unless if you get stains in it, then wash it, but hang dry it so it doesn't shrink.