My advice would be to learn form. Form when you lift weights, form when you wrestle or grapple and learn the correct way to throw your punches. Train form until you don't even have to think about it. I wrestled in high school and tourneys in college. It seemed I was always on the small side. Nothing is better than beating a guy more muscular than you because you trained.
Your body will decide when the size will hit. If you train correctly you will always be in shape no matter what your size. When the muscle does start packing on, your form will double your efforts. You can always pack on more muscle at that point but learning form is the most important because it takes a long time.