Not sure if you have a local music shop, but the best thing would be to take your horn there and ask to play a few different size mouth pieces and to help get the idea of what is the right size for you. Student horns get 12C mouth pieces for a reason... because they are easy to learn on for 12 year old mouths. When you get past 16 or 17, your probably got bigger "features" than you did 4 or 5 years ago. So, you need something bigger to give your lips room to work. I went up to a 5G in high school and later to a 6 1/2AL after college. While I did this primarily to extend my range down, it also brought my control in high registers in check.
Generally, the things to look for when testing for mouth piece size is going to be comfort and tone. You want to be relaxed when you play the different ones. Being relaxed will help you find the best compromise between tone and comfort.
After you get your new mouth piece, practice a lot with it. It will take time to form the proper embouchure on a new mouth piece.
~DB