Thor's Might is a house-ruled cleric spell that can augment the caster's size and power. It seems to combine enlarge person with possibly other buffs such as bear's endurance and bull's strength. It also seems to only be able to target the caster. It is likely that Thor's Might is a renamed version of Righteous Might, a level 5 cleric spell.
It seems to have a duration based on level: when Durkon used this spell when overwhelmed by Goblins (because of Belkar's irresponsibility) in Xykon's throne room, it only lasted for a few rounds, while it lasted throughout a protracted battle in Cliffport. This might be only for story purposes however, and Durkon could have recast it on himself "off-panel".