Also, unfortunately with breeding you can't always guarantee sizes. It's not like you can mix a small horse with a big horse and it comes out medium. Even if you breed to a big stallion, there is a possibility the horse will still be short like it's mare.
Also, if your mare is small, and you breed to a large stud, you may run across the problem of the foal being too big for the mare and have a LOT of health concerns for your mare. Think of it this way, would you breed a big Rottweiler to a female Chihuahua? No, because the puppies would likely kill the Chihuahua before they were even born. A foal that is too big for it's dam can cause all sorts of internal injuries.
I really wouldn't risk it, especially for a foal who will also be grade. You may end up losing them both.