Actually, it depends on WHICH canon you're talking about. As I recall, MT uses 1.5 meter squares, and says 2 squares equals a Dton. That's a ceiling of 3 m. That's with 13.5 kl Dtons.
I think TNE and T4 use 2 meter squares and 14 kl Dtons, which means a Dton is 1 square, 3.5 meters high. (If we go with the true value for a Dton, though (13.5 kl), it's a much more reasonable 3.375 m.)
Having lived on a ship for several years, I can tell you that a distance between decks of 3-3.5 meters is not as crazy as it sounds. There's lots of ducting and wiring all over the place (and if its military, there's no plastered walls to cover it up, either) and you usually not touch the ceiling, except when you're going through bulkheads (walls). I forget the technical term, but the openings (some covered with hatches/doors) require you to step over a lip and duck at the same time, unless you're really short, and then you only have to step. They're made like that to contain flooding and smoke, btw.
I think civilian luxury staterooms are designed around the motel room concept. I have no doubt that cruise ships will offer these large rooms.
Crewmembers and even officers will not have this much room. (Exception: If the ship is big, any commanding officers, executive officers, and fleet commanders will have large staterooms, but half of that space will be occupied by an office that they work from. On my ship, the CO, XO, Admiral, and the CO and XO of the Marines had large staterooms like this. This was a large amphib, with lots of Marines.)
Regular ship's officers shared something smaller than a small stateroom with another officer. (The top 5 or so - the command crew - had their staterooms all to themselves.) Marine officers had the same size room, but squeezed 4 people in the same room.
Crew didn't get that luxury. We had berthings. Each ranged in capacity from 20-300, and you slept in the same berthing as your department. (A department would be Combat Systems (big), or Navigation (small), or Engineering (really big).) Image a bunk bed, but with three beds on it. The stack was about 7-8 feet high; the top bunk at eye level for me. Most people had to stand on lower racks to get into top-racks. These bunks are placed close enough together that you cannot sit up in them; you've got about 1 cubic meter to sleep in.
These are called "coffin racks", btw, mostly because of the confined space. Put two of these 3-packs close together, maybe 18 inches of space between them, and you have a 6-pack. It takes up about 4 square meters of floor space and as I said, reaches about 8 feet high. They don't touch the ceiling, but they do often bump into deck-bracing for the upper deck and for the ventillation and wiring clusters.
The Marines had it slightly worse; their racks were stacked four high in most places, but the deck they were on had a higher ceiling to accommodate that. Still, to get to the top-most rack you have to climb for a minute. Bring a harness! (And don't fall out of your rack either! Some ships had a little catchstring for you, but ours didn't!)
Okay, feeling cramped yet? Think Traveller asks way too much for sleeping space? Well, I'll add to the above a set of toilets and showers (the Head), about 1 stall of each kind for each 20 people in my berthing. Smaller berthings had at least 2 stalls apiece. There's also a small "common area", where we could watch TV or play Spades. Enough for a couple dozen people to enjoy.
DON'T consider all that extra space Traveller says you should have as life support. It's not. Life support is already paid for. For the most part, you can ignore plotting LS on your maps.
But what the remaining space MIGHT get taken up with is things like the Mess Decks, the Galley, and the passageways and ladders in and out and around the berthings. We had a big mess decks, capable of handling about 200 people at a time. There's also food storage, but that could be considered LS, since there's just so much of it.
Anyway, this should help you in designing deck plans.
