I heard steroids make penis smaller, i am wondering what if the user already has a small penis ?

Not so: They will make your testes smaller. If you have any social life, word will get out fast that you're cheating and breaking the law. The vast majority of women prefer a naturally-fit man rather than a steroid lunk. The greatest danger is that you'll do something on a steroid-induced impulse that you'll regret for the rest of your life. Don't do steroids, man, but just work really hard in the gym.