Look for different ones- a size difference of almost two inches will be too much to make them worthwhile IMO. I'd look into kids sizes- they'll probably be too short technically, but I'd buy too short by a couple of inches instead of too tall!
Here are a couple that you may want to try: Children's Dublin Easy-Care Half Chaps | Dover Saddlery
^^ I had a pair of these in the adult size... but a warning- they're not extremely durable. Fit may be your biggest concern though!
These would be my recommendation, unless you're still growing. If you're 14 years old and still have some filling out to do, I'd buy the above. If you're pretty confident that your size will not vary too much then I'd recommend these. I LOVE my ariat half chaps, and they've lasted me about 10 years! The kids large tall should fit your fairly nicely, just may be a bit short. Ariat® All-Around III Chaps Kids™ | Dover Saddlery
I'd also double check your measurements to be sure, and make sure you have someone else do the measuring. And, of course, measure over all of the pants/socks/etc that you plan on wearing so that the chaps aren't too tight when you're fully dressed!