I am not familiar with all the different ship design systems, but I am curious about how much they vary across editions.
As noted, the combat rules of High Guard suggest 200K as a "soft" maximum size for TL15 or so.
Thinking about Traveller beyond the scope of the Third Imperium, ships
qua ships don't get as big as they do in, for example, H. Beam Piper's
Space Viking series, or Star Wars. Or Babylon 5, though I may be wrong there.
Space structures -- things too big to be treated as ships -- on the other hand, can be enormous, since we do have orbital starports and stations of all types, including Ringworlds and Dyson Spheres. And there are jump-capable "structures"; for example, the 50 billion-ton Loeskalth planetoid, a TL9 mobile home and base for raiding. One might be tempted to call it a ship, but it's really more like a world than anything else. For one thing, its first purpose is as a habitat. For another, attacks on such a thing are more likely to resemble planetary assaults than anything else. And that sort of distinction is convenient. (Footnote: at high enough TLs you can jump
planets).
The cutoff, then, is some fuzzy line in the millions of tons. I will make bald, relatively unsubstantiated statements now:
1. The largest "ship size" in Traveller is somewhere between one and ten million tons. Alternate limits (e.g. Book 2 or Star Wars) are variant.
2. Larger structures are more correctly treated like "worlds" than ships.
There are fuzzy lines here. For example, a "crew" in the millions clearly operates more like a population on a world, while a crew in the thousands is no problem for us to understand (e.g. supercarriers). It's probably safe to say that a "crew" of 100,000 is better treated as a "population" of a "world". But what about 10,000 people? Is it a crew, or a population? So population is one important consideration, but not the only one.
