I wonder...if we made the (baseless, mind you) assumption that the torpedo launcher on the JJ-prise is the same size as that on the refit...how big that would make the ship.
Which points out a problem with using *one* feature for a sizing reference...depending on which feature you use, you get different sizes. (Though if you have a human figure in the shot, that helps narrow it down a bit.)
It seems obvious to me that the ship in the movie appears to be different sizes in different scenes...so in my imagination, I'm gonna put it as maybe 1/2 again as big as the TOS ship. Maybe a wee bit less...1/3 again as big. But that's just a personal interpretation.