D&Dtools1

As mighty wallop, except that the bludgeoning melee weapon damage increases by one size category for every four caster levels you have, to a maximum of five size categories or up to an effective size of Colossal.
For a Small weapon that increases by more than one size category, begin by referring to the table in the mighty wallop spell description.
Once the weapon has been adjusted to an effective size of Medium, or if it was already a Medium or larger weapon, refer to Table 2-2: Increasing Damage by Weapon Size (DMG 28).