Mighty Wallop Greater (RotD):
"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 fi ve size categories or
up to an effective size of Colossal."
Note the cap of Colossal.
Also note, I'm not trying to be annoying and I want this trick to work. However, any "trick" I use will be heavily scrutinized by my usually-close-minded DM so I have to have strong and thorough justification for things. In the example above where I have Shapechange and Giant Size, it would make sense that Righteous Might would increase size further, but other than the flavor of being twice the size it is not clear to me that there is any game mechanic benefit of extra size beyond Colossal+.
Another place to look is Deities&Demigods (granted it's 3.0) under the Alter Size Salient Divine Ability. It allows growth to 1600 feet tall, but is still only considered Colossal (not even Colossal+).
I'm hoping someone can convince me there is a way to gain size benefits past Colossal+, but so far it sounds like that would be a house rule.