...will my decrease in size?

In high school I was a sprinter, 5'2", and about 105. Running is a very healthy exercise, and although it might make your smaller, if you stop running they will "grow" back to their original size. I'd go for it . . . well, I did go for it :) Plus, maybe you'll meet a nice boy who runs track and doesn't care so much about your size as your awesome athletic ability!