As mention before the typical ton is 1.5m wide x 3m long x 3m tall (1 square x 2 squares x 2 squares on graph paper) That makes ship decks almost 10' clear from deck to deck, leaving about 8' of headroom in between floor structure/mechanical chases etc...
For game mechanic purposes the difference between meters and feet are basically equivalent when looking at GURPS vs. Mongoose.
1.5 meters = approximately 4' 11 1/16", less than 1" difference. I just say "about 5' x 5'".
True volume, size, and scale in a "real world" scenario would matter, but since this is a sci-fi game, don't sweat the small stuff. GURPS game mechanics is based on yards, that's all there is to it.. But as a GM fiat you can simply say "GURPS is based on meters" and be done with it. So instead of a hex equaling a yard, you could just say a hex equals a meter.
I'm an Architect so I pay very close attention to not only deck plans, but also sections, profiles, etc... In the very early days (Jr. high School) I would make deck plans on graph paper, then realize how "thin" they would be after trying to sketch elevations of them, and then completely redraw the thing.
Also I would try to break your plan into specific functions if at all possible.
Plans have improved greatly over the last 10-15 years or so (affordable over the counter 3D modelling programs is my guess) but in the early days deck plans were a mishmash of bizarre poorly thought through designs. I've seen state rooms placed in all kinds of weird places, opening directly into a jump drive, or directly into a cargo bay "cause that was the only space left where it could fit" Just imagine if a stateroom were crammed into one of the Warp Nacelles on the Star ship Enterprise from Star Trek... Thats kinda what happened in the real early days of Traveller. lol..
Look at current examples. Break your functions into cargo, engineering, command/crew, passengers, Control, etc.. Try not to place passenger state rooms where they mix with crew staterooms etc... (or enlisted men/bunk rooms VS. officers etc...)Think of a modern cruise ship, or an cruise liner from the early 20th century. You wouldn't want your paying customers to mix with the staff if you can help it. You wouldn't want to have passengers have to walk through the engine room to get to their stateroom!
On very small ships, this becomes less of an issue.
Also, don't fall in love with your first layout. Make at least 3 or 4 and keep playing with the design until you find something cool and different. Start with basic sketches on blank print paper, then transfer to graph paper (or computer software) only after you get a couple of good ideas.
As mentioned in earlier posts, a 4 ton state room includes common space such as corridors and common areas like galleys and passenger lounges. The most common actual size for a stateroom is 3 tons (6 squares) but in GURPS and in T20 I think... An actual stateroom is drawn as 2 tons (4 squares) with the other 2 tons in corridors and common spaces etc... Also most rules allow a 4-5% or so "fudge factor" when drawing deck plans. As a rule, as long as your write up says you are a 200 ton ship with a M2 and J2 with 8 staterooms or what ever, nobody will care much if the actual deck plan shows 206 tons as drawn (because you might have too many "squares" in corridor space or a galley etc... ). It simply doesn't matter that much for a Science Fiction role playing game. You are not working for NASA. Only your friends so you guys can have fun on the weekends. Just make it as close as you can and have fun with your designs.
