Over on Memory Alpha, it says the finalized length of the Enteprise is 2397.75 feet (725.35 meters). That's reported in the "Starship" Blu-ray feature. It also says this size was specified by the licensor of the model kit, who got it from ILM.
As many others have said, the article notes that the initial 1200 foot size was considered too small as production continued, so they scaled things up. By fall 2007, the size was wavering at anywhere between 3000 and 5000 feet.
Full link here. Scroll down to "Size" near the bottom of the page. It is quite well documented.