Masturbation does not change your penis size.
Men worry about penis size unnecessarily most of the time. Are you sexually active? If not, what would your penis size matter right now? Even when you do become sexually active, most women do not even care about penis size. Who would want to be with a woman who only cared about penis size anyway?
A lot of guys watch pornography, then think they are small because they do not have a nine inch penis like the guy in the porno. Penis size varies a great deal, but the average is 5-6 inches, although smaller and larger are certainly not unheard of.
You are still very young, so you might notice your penis growing a bit still over the next few years.
Either way, if your penis is longer than about three inches, it will be able to do what it is meant to do. People are different, and that's ok. There are very few men out there who should be worrying about penis size.