There's a Star Trek article in Cinefex #118, with this quote from ILM Art Director Alex Jaeger
BINGO! Finally some conformation about the upscale/super-size. This also seems to confirm the ship may have still been at 1,200ft when the efx were done for the construction sequence.The reconfigured ship was a larger vessel than previous manifestations-- approximately 1,200-feet-long compared to the 947-foot ship of the original series. Once we got the ship built and started putting it in environments it felt too small. The shuttle bay gave us a clear relative scale -- shuttles initially appeared much bigger than we had imagined -- so we bumped up the Enterprise scale, which gave her a grander feel and allowed us to include more detail.
Now I wonder if this may be leaving a loophole for the ship to be scaled back down.
Hmmm....interesting...great quote though. In my mind this is how big the ship is. Its really the way it was intended to be which is evidenced by the external appearance of the new E.