In that thread I posted, I didn't see anything about what I ended up doing in my own campaign. So for what it's worth, I ended up making a fairly significant change to the spell effect:
Size changes spells rely on the existing rules for giantism in Creatures and Treasures, affecting movement, OB, and DB. I decided that changing +/-30% does not modify any bonuses (really, no significant "size change"), +/-40-60% is a single size class change (+/-20'/rnd, +/-20 OB, and -/+10 DB), and +/-70-100% is two size class changes (+/-40'/rnd, +/-40 OB, and -/+20 DB). I also incorporated a rule from another system that says gear changes with the target, but if it leaves contact with their person, it immediately reverts to normal size.