I'm skeptical about granting one PC free extra abilities based on size when the rest of the system hasn't been designed accordingly. That said, if you look at RMU, there are rules for the size of the rock or weapon (x4 weight per size increment), the damage (x1.5 for one size increment, x2 for two, x3 for three, etc), the critical adjustment (+1 severity per increment), and the range (use the same modifier for damage on the range increments).