Engines and fuel tanks/fusion reactors might take a lotta space (fusion reactors a LOT of space, what with the necessary heatsinks, insulators, baffles, and cooling devices, never mind radiation shielding)... but from the looks of the techroom animations, they've miniaturized super lasers a good deal by then, and I'd have no doubts you could have microboard CPUs and miniaturized sensors a few centuries in the future... So basically, you get about four to twelve cubic yards of engine (for a smallish ship), maybe a cubic yard of everything else, and two or three for the pilot to sit in, likely nested among all the components rather tightly (help dampen G-shock on something so fast as a Dragon, anyway). With a Shivan, no life-support necessary, so there you go. Plenty big enough.