Could always use one torp room two launchers like Shane Johnson did...
The problem with that is that this isn't consistent with what was seen on-screen. I mean, we see the port-side torpedo bay room take phaser fire, basically be blown to the proverbial "smithereens," and then in a very short period of time later, we see the Enterprise firing torpedoes out of the starboard-side tube... and what is presumably not more than a day or two later, we see Spock's burial tube fired out of a clean and undamaged torpedo room, with the tube being fired through the starboard launcher.
It seems pretty clear from that on-screen evidence that there was a port-side and a starboard-side torpedo room, and that the port-side one got shredded while the starboard-side one was left effectively intact and operational.
As for "how much larger" the on-screen one was than what we see here... I suspect that the answer is "not much." Maybe 10% or so? The ship is much larger than we sometimes thing (and, at other times, actually quite a bit smaller). It's this whole "scale misperception" thing that makes 3D modeling of a ship so fascinating. When dealing with 2D drawings, it's easy to miss things (like ceiling lines... something I think Strategic Designs forgot about on occasion in their blueprint set). Or with issues like which deck Engineering really is located on in the TMP ship (I suspect that it needs to be maybe two decks lower than usually assumed based upon Probert's concept sketch and Kimbel's cutaway)
In a case like this, I'd be inclined to look more at "what was the room intended to do" than anything else. We see the following going on in there:
- A small group of cadets being inspected. How large of a room would be required for that?
- Torpedos being loaded for battle. How large would the room need to be for that?
- A funeral. Based upon the number of attendees, how large would the room need to be for all of them to have been present, as shown?
It might be interesting to pull up a 3D presentation of a person and place it in that scene... or several. I'm not sure how effectively you can do that in AutoCAD, so you could always just create a very simple "box" representing the approximate volume occupied by a standing human male, if having a "real person" figure isn't really an option.