I'm 15 years old and I'm really worried about my penis size. It still looks immature. I have a brother who's 13 and his penis size has increased more than mine. I have searched the Internet about it and there's no help. My penis is about 5 inches when erect. I have a masturbation addiction, but I have stopped as from today. Every time I used to masturbate my penis looks really small and at times it looks different (kind of bigger). I'm 16 this year, and I really want to have sex with my girlfriend, but my penis size isn't enough. Is there some kind of special method I could do to make my penis mature faster?
So why are you aiming to father a child at 15? After all, sex does tend to get girls pregnant.
The normal range for an adult penis is between 3 and 8 inches erect. The average is 5.2 inches [http://www.bjuinternational.com/bjui-blog/normal-review-analyzes-data-flaccid-erect-penis-lengths-men/]. So you are well within the normal range, even if you are a bit below the average. Not knowing where you are on the Tanner Scale, I can't say if it will grow anymore or not.
Every person is different. There are different shoe sizes, nose sizes, ear sizes, and penis sizes. Even two people in the same family will end up with different sized body parts. It is all based on genes. Thus the size of your brother's penis says nothing about your penis size. Your own penis size varies because it inflates like a balloon. The size depends on the strength of your blood pressure at the moment.
Penis size has little to do with sex. Your future wife's vagina stretches to accommodate whatever size your penis is. Sex is more about technique not equipment size.
But let's go back to this desire of yours to commit fornication. I find it interesting that you want to give up masturbating (I assume because you think it is wrong), but then turn around and talk about sinning with another person. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). See: Waiting for the Proper Time.