Medium will be fine for you... they are pretty generous size wise, and that does 15-25lb in theory, but probably more in reality.
Not got the guitars in that size anymore, but think we have the rest still, so you should have plenty of choice. Just click onto medium and it will tell you if we have it or not. I would check, but just packed them all into a box for a baby sale tomorrow! So if you do see one you like either go for it, or let me know so I don't sell it there....
Right, off to bed! Night all xx