IMC, Max Composite Bow Strength Modifier, or MCBSM scales with size modifier with +4 being the base for medium sized. So it goes Medium +4, Small +2, Tiny +1, Dimunitive 0, Fine -1 and Large +8, Huge +12, Gargantuan +16, Colossal +20. I also allow the characters to add the 'mighty' descriptor...