Take a look at the gantry crane heading into the lower saucer from the left, just above Kirk's head. You can clearly see two people walking down its length, especially in a higher resolution shot. Their relative size indicates a similar deck size and layout to the refit Enterprise.
What are you basing your estimate on? The ship was originally designed to be about the same size as the refit, and many effects shots reflect this, not to mention the window and airlock size. There's also the matter of Kirk's escape pod. Yes, a few shots (shuttlebay, bridge window pan-out) indicate a 700-some-odd meter length as well. But as both are seen, both are valid. I prefer the smaller size because it doesn't throw all other starship scaling relative to the Enterprise out the window. How is this "delusional"?
The gantry isn't touching the saucer. Move that person and make him stand against the hull and he'll be a lot smaller.
What am I basing my estimate on? Eyesight and common sense. 300 metres is nothing. Look at all the structures below. The ship is massive.
You should be less concerned about canon and how the ship fits in with other Starfleet ships in other universes and more concerned with a) official statistics and dimensions showing it to be ~700metres, b) on-screen evidence and c) common sense. Look at the under construction shot, look at Kirk and Scotty running through the enormous engineering set, look at the shuttle bay etc...
I prefer the smaller size because it doesn't throw all other starship scaling relative to the Enterprise out the window.
It's a different universe. Kelvin attacks. Starfleet makes BIG ships.
Abrams has changed enough visually to show he wouldn't have a problem making the ships themselves a differnet size.