Here's some context:
Naïve math puts a cube of a billion dtons, assuming14 cubic meters a dton, at 2410 meters on a side, or about a 3 km diameter sphere. Not really that big, if you're thinking habitat size. In Behind the Claw, the Lurent planetoid ships are mentioned as holding tens of millions of Lurent (who need 6 times the space of a human), so I imagine they're in the tens of billions of tons.
A long time ago in a context not related to Traveller, I used a quick estimate of a million people per cubic kilometer as possible density crammed into a 3D, city-like habitat - think Manhattanesque cubes with all the building 300 stories tall. - so that would be 14 million crammed into a billion dton "space city".
A spinning habitat would have a population more related to surface area, so at let's say ~ Netherlands population density, you get 1000 people per square kilometer. Assuming only one level of habitation, a stubby cylinder (actually needed for spin stability) that held a million people would have, for example, a 16km diameter and a 20km length (not counting any endcap habitation). You'd probably do better if you put an under-level of suburbia density below a farmland section and put business/industrial sections in the end caps or tall towers. The dton volume of what I just described would be about 287 billion - and could hold 1 million people per 'level'. On the other hand, if outer hull was 1 km thick, we get back to the cubic calculation above and you get over 900 cubic kilometers - so we're pushing closer to a billion than a million - okay, no, I don't know how you would feed them all.
Another example: Phobos, inner moon of Mars - according to wikipedia at least - is roughly 27 x 22 x 18km and has a volume of 5783 cubic kilometers (413 billion dtons) . At 1 megaperson per cubic kilometer, that's 5.7 billion people - again with food supply assumed as an external problem or using a very very good and tight closed recycling system. But in Traveller terms, at that density, each person gets more than 70 dtons allocated to them, which is more than adequate for long term survival.
To Alqualonde's comment, yes, the highport, industrial regions, maybe even habitats for highport employees wouldn't necessarily be part of the same complex. Some of the capacity would be in a downport, as well. The Third Imperium's rules around extraterritoriality, however, would likely tend toward centralization of starport capabilities- at least the docking and commercial aspects - into a defined locale. I imagine navy and scout bases might be entirely separate, unless the navy want's to share defensive capabilities with the port authority.