To be honest, I really don't see a need that every next-gen Enterprise needs to be bigger than the previous one. Unless you wanna eventually end up with ships having city-sized crews in the thousands, with both officers, enlisted and civilians, and thus making it a very tempting target for enemy ships.
The Sovereign Refit should remain "relatively" equal size to it's standard version, but ofcourse certain additions can be made, such as increasing the saucer height, or extending the stardrive area (both of which you've done). Also, consider that removal of certain "warlike" features, would result in increased usable space aboard ship, without necessarily expanding the ships size.