There is a size minimum, not wure what it is though. My shot is, I think the biggest one out there-it is also the cheapest. There may be some issue of wind resistance, but I done think its a big deal. I mean look at the world class guys, they use big rocks... I think the biggest thing with the size of the shot is how big of hands you have.
I have real big hands and the bigger implement fells more comfortable and stable in my hand.