You are a perfectly normal size, just a bit longer than average. You are certainly plenty big enough to be able to have good sex and satisfy a girl - that has everything to do with technique, and not a lot to do with size, unless you are very small. Do not judge yourself by what you see in porn (if you do ever look at porn) - most of these people are freaks. There are men on this board who have porn star-size dicks who wish they were smaller, as it hurts their girlfriends too much when they have sex.
Masturbation has no effect on size whatsoever. Doing it more, or doing it less, won't affect your size either way. So you might as well not give up - don't deny yourself the fun ;-)
Don't pay any attention to adverts on the 'net for pills, vitamins, potions, creams etc. that promise to make you bigger. They are all false claims, trying to make money out of men's feelings of insecurity, or trying to convince them that they could be "more of a man". It is nonsense, none of them work, but you are right to be worried about side-effects. I don't know what palmetto is, I doubt zinc or Vitamin E will do you any harm unless you are taking way over the medically recommended dose, but they will not make you any bigger.
The only genuinely effective method to increase size is surgery, which is expensive, painful, and not always successful.
Some men on this forum say they have had limited success with various stretching exercises, but from what they've reported it seems to me that a great deal of time and effort is required, and the claimed increases have not been that big anyway. Have a search on the forum and you should find a couple of these topics.
But honestly, you are a perfectly good size, you've got nothing to worry about!