If you're thinking about spacecraft dimensions then you're probably thinking of drawing deckplans someday, which means that what you're really thinking about is how many little squares on a piece of graph paper equipment of a certain tonnage should take up.
Personally I prefer using the K.I.S.S. method to figure this out (Keep It Simple, Steal from traveller.)
In Traveller a single square is 1.5 by 1.5 meters, with the decks being 3 meters tall. Each square is 1 ton. This was based on liquid hydrogen having a volume of about 14 cubic meters, so the 1.5 x 1.5 x 3m decks (living space) plus about 3 meters of deck material worked out well.
It works out well for Battletech too for a couple of reasons; A standard Map Hex is 30 meters wide, and a standard Level is 6 meters tall. Both numbers divide into 1.5 m cubes nicely. The tallest battlemechs are also 12 meters tall too, another nice coincidence. Make each standard deck on a ship 1 level tall and you get 3 meter thick decks with 3 meters of living space above them, with equipment taking up one ton per square. Even better, you don't have to draw things like plumbing and air ducts, as these are all located within the deck itself. Larger decks, like those containing battlemech bays should be two or three levels tall, 9 meters is plenty of headspace for small mechs, and any 'mech can fit in a 15 meter tall bay (remember only one of the decks is there, so it's 3 meters of deck material plus 9 or 15 meters of open space above it.)
Under this system things on a standard deck are 1 ton per hex, while a taller deck can hold 2 or 3 tons. This makes drawing things easy; a first class cabin is ten tons so it takes up 10 squares, say 5 x 2. A 'mech bay is 150 tons so it takes up about 75 squares on a 2 level high deck, so call it 10 x 7 squares with 5 squares left over for the bunk space provided for the pilot and tech.
But what about all that extra space for corridors and ramps and galleys that the fluff says takes up most of the volume of these items? I say just draw it as big as you want. It's empty space so it doesn't mass much, and BT spacecraft are famously less dense than empty soda cans so go to town with it (Looking at the art dropships seem to have a density of about 80 m3 per ton.) You can base the additional volume for this space on the ships' description - a small ship with cramped crew cabins probably has a tiny galley with fold-out tables whereas a large liner with 1,000 first class passengers probably has everything from ballrooms to bowling alleys (is zero-g bowling fun? I wonder...) This also works out for the empty space in dropships like the Union. Those ships are used as mobile repair shops as often as transports so it makes sense for the ship to have a huge internal space for the techs to work on the 'mechs. Finally, those immense warships probably have equally immense areas of empty space within them; the fluff states that the armor consists of multiple layers separated by empty space and it makes sense to put a fair amount of space between the internal equipment too (you don't want an exploding laundry machine taking out the fire control computer do you?)
One last note regarding cargo bays: These should have a lot of volume, much bigger then their tonnage indicates. This is because spaceships unlike (water)ships don't sink if you overload them, they just accelerate slower. It should be perfectly acceptable to load two or three times as much stuff in a cargo bay as it is rated to carry, as long as you recalculate the thrust rating of the engines. I've never seen that mentioned in the fluff or rules but it probably happens in the "real" BT universe all the time. After all, a mercenary unit that captures 5 clan omnimechs isn't going to leave them behind because their Leopard is full, they'll stack them in every available open space and then just pray that the ancient ships engines can still manage to reach orbit.
