I think it's deliberately left open. The campaign comes with a map, which shows the main level in abstraction and we read of "gravity dungeons" down in the belly of the beast. But that's about it.
Given the size of the map and applying a guesstimate scale to it (no actual scale given), I'd say the main dome and thus the main level is between 300 and 350 metres at its widest point. That would give us a ballpark figure of 4,500 to 5,000 dtons for that deck, if the ceilingwould indeed be just one level (i. e. 3 metres) high, due to the oval shape of the deck (very rough, its oval not a perfect ellipsoid). The ceiling varies however, since the star port and the royal docks as well as the great hall and the gardens will have much higher ceilings. Essentially, their internal volume in dtons will be much higher and oriented vertically as well as on the X and Y axis. A plethora of domes, additional decks and substructures further contribute to the massive size of the construct.
Given that the structure is described as massive but very cramped and the majority of it will be engineering and substructures, I'd personally ballpark it into the 30,000 to 50,000 dtons area. So, up there with small capital ships such as cruisers. Essentially, imagine a planetoid monitor (50,000 dtons, High Guard, p. 184) with its m-drive, j-drive and the spinal meson cannon stripped out and their space reused for bunks, hyrdoponic gardens and the storage of food and perishable items for 70,000 people.
Well, that's my take on it anyway.
