I think the problem is that they DID know, but everyone had a different intent from one another. The exterior looks workable at a scale close to the TOS/TMP ship, considering the windows and the hatches, but then there's the construction scenes and the shuttlebay and that darned shipboard brewery.
Think about this for a minute. The TMP Enterprise had that enormous cargo bay that was large enough for shuttlecraft to fly around inside of it dragging big trains of containers behind them; based on the cutaway designs, this takes up about half the engineering hull with some room above and below it for engineering decks and parts. Forward of this, there's the engineering gear with five or six decks of vertical intermix and a whole bunch of other crap, like a small office building packed in behind the navigational deflector.
Now in the NuEnterprise, we don't see anything that could give an indication of scale between the interior and exterior, but if you assume that someone scraped out the TMP hull completely, ALL of that engineering gear from ST-XI could easily fit inside that cargo bay and the gutted space of the engineering compartment, especially if you assume that--for some reason we're not privy to--Starfleet intended the shuttlebay, cargo bay and engineering compartment to be really just the same space with little division between them, and there might even be some functional overlap between them.
Think of it as a kind of industrial optimization. Why have engineering parts stored in the cargo bay when you could have open space in the actual engine room to store pallets of spare parts and engineering cargo? take an example from the TOS engineering set: take down the chainlink fence that separates the engine bay and then add a catwalk down the middle, and what do you have? It would probably look pretty big if you didn't know exactly where you were, and we (the audience) do not. In that sense it bears a pretty strong resemblance to, say, the engine rooms of
modern ships, which have the same cluttered industrial look and look pretty freaking huge to landlubbers who don't know their way around.
Furthermore, there's a certain logic to this that I particularly enjoy. There's clearly some modularity going on here: the Saucer section contains ALL of the habitable volume of the ship, or at least, any place where the crew would WANT to be when they didn't have work to do. The engineering compartment, it's just a big factory/warehouse/hangar bay so utilitarian it doesn't waste precious room on things like walls, blinking LEDs, chairs, hell even the turboshafts are just big columns standing in the middle of the deck with doors on the bottom (not unlike the shafts in the TMP cargo bay). My only complaint is the obvious hazard of what happens if someone should breach the hull of the engineering compartment, but clearly there's a certain amount of failsafe hardware built in for that kind of emergency.
So, somebody did think it out and make instructions, but the problem is that everyone wanted to think it out and do it their own way. And now we have a gigantic ship with docking bay-sized windows. Or Starfleet reverse-engineered a TARDIS.
Well, the new Enterprise seems to be quite a bit larger than the original, based on the size and location of the "viewer window" on the bridge. Incase you missed it, the window on the exterior model is in almost exactly the same place as the "sensors" in the TMP Enterprise, and the same size to boot; since this is essentially a floor-to-cieling window on the bridge set, this gives you a sense of what the deck height actually is, as does the fact that the Enterprise' bridge is apparently accessible by corridor as well as by turbolift.
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