Khisanth the Ancient
Explorer
Possibly. The top Primordials might be planet sized...though I don't necessarily consider Ymir amongst the top.
The Earth and various heavens were made out of Ymir's body, so it seems that he would have to be at least planet sized. I agree that's a bit far up for the progenitor of just Frost Giants though; that's why I suggested making Ymir progenitor of all of them (& maybe the first Primordial) and giving the Frost Giants to his son Thrudgelmir (who was a six headed monster).