Don't be scared about your height. It's going to be what it's going to be. I know lots of pitchers who are tall and slow and some who are short and fast.
It's a 5 ounce ball, as long as you are strong and fluid you will eventually bring it with the best of them.
My son has been on the shorter end of the spectrum and yet throws harder than anyone in his school and I have only seen one kid in his league throw with similar velocity.
79 as a freshman is above average. Many pitcher you will face will barely be at 70. Anything over 75 as a frosh is above average.