Depends on who you believe. I don't think physical size and muscle equates to velocity. There are just too many guys that are small and lanky (Nolan Ryan, for example) that can throw as hard as anyone else. If you want to become a better overall pitcher, I think a good program could help you. Clemens is also a good example of a pitcher that has benefitted from being in tremendous shape.
I would take to Mike if you want to add muscle/ strength in a way that will aid you performance in pitching.