Now that the movie is out in HD on iTunes, let us return to the "how many decks are in the saucer?" issue. It turns out to throw a monkey wrench into things. We get a really good view of a corridor through a hole torn in the saucer edge at about 1:40:13:
(Click for high-rez)
I've brightened it up a bit so you can see things more clearly and outlined roughly where the corridor meets the saucer edge. As you can see, at least in this image, the saucer windows are not
floor-to-ceiling like the bridge window. There's only room for two decks plus several feet of service space above and below. If you eliminate the service crawlways, you can squash three decks into the saucer, but then they don't line up with the windows at all.
It looks like the ILM guys scaled the corridor to fit between the inscribed "art deco" lines, resulting in a shot where the ship has to be smaller than 725 meters.
As he is the unofficial official documenter of these things, I leave it to King Daniel
for further investigation.