Yeah, I know. The scales of both the Vengeance and the Enterprise change several times. In the first movie, when Kirk is looking up at the ship being built you can see several people on scaffolding which allows for a size comparison. I calculated that it was somewhere between 300 and 400 meters long. Then of course there's the shuttlebay shot which they scaled it up for. After that it seems to revert back to its original size and stay that way for the whole movie. You can even see that the rim of the saucer is only two decks thick.
In STID, it also seems to be around 350 meters or so. And the Vengeance is twice its size, until the crash scene where it seems to grow. Since it seems to be rather inconsistent I just choose the more plausible option of having the smaller sizes.