"Eat the flesh of Gods, become one of them."
Also, you could model the class ability on Expansion (PsyWar manifestation). However, since you already have the idea how the actual growth works, and make the size increase permanent and real, the only advice I could give you is to focus on both benefits and drawbacks of size increase over levels. What I mean to say is, it is very likely that the size penalty to DEX would be larger in the beginning but drop as the character grows accustomed to his/her new body. So, maybe you could first make the arms grow in size (grants reach but also gives -2 on DEX without adding to STR), then make the whole body grow (giving +2 to STR, and further penalizing DEX) which might not be capstone ability. Capstone ability should maybe be "New size adaptation" which would round up the class by equalizing bonuses/penalties. Also, 10 levels might be too much for a PrC that focuses only on that one thing. Maybe 5 levels is a better choice?