Hey Krust, or even anyone else if they think they can answer this...
What level, or what Spellcraft DC would you set a spell that causes the target to grow to the level of their virtual size catagory? That is, cast it on a human with an inherant strength of 30, and that human suddenly becomes large (and thus gains -2 dex, +10 str, +4 con, +3 nat armor, -1 AC and -1 to attack). Obviously the restriction would have to be added that it could only be cast on an individual target once at a time, otherwise the +10 to str would provide another virtual size catagory, allowing it to stack infinitely with itself all the way up to universe size levels and beyond.
I want to give this as a Spell Like Ability to an epic/immortal race I'm designing (self only), and want to know how it would likely effect CR. The race would come in age catagories, and though it would only have a natural size of medium, each catagory would grant it +20 str, and from the beginning it would have this SLA.