The 'referee' section of the original Traveller set advises that a good way to start a campaign is to generate and detail a single subsector of space suitable for adventure. This methodology has become much less common since the rise of the official 3rd Imperium/Charted Space setting. Most folks just plop the PCs down into the Spinward Marches or some other official sector and cut loose.
Traveller enthusiast Ken Pick refers to the pre-3rd Imperium phase as the Burgess Shale Period of Traveller. I find Mr. Pick's retelling of the linked Shavian Empire/Foible Federation campaigns to be very compelling. You can read about them here. Elsewhere on Freelance Traveller Mr. Pick makes a case for returning to the one or two subsector model. He explores the idea of setting aside the multi-sector Imperium and High Guard's million-ton ships to instead tackle "One subsector (maybe two), TL 11 (with TL12 being gee-whiz cutting edge), exclusively using Book 2-designed starships. A "cozy" campaign universe -- with 15+ years of hindsight, probably what Original Traveller did best."
Well now somebody has taken up this challenge, and with real style. RPGnetter Methuslah recently announced his 24-Hour Traveller Campaign, whereby he develops a single subsector for use a complete Trav setting in the space of 24 hours. His starting material was "Exonidas Spaceport", a nifty little Traveller article published in Dragon #52. Around this focus he built a subsector-sized campaign during the last day of 2005. You can download a PDF of Methuslah's setting, called The Sky Aflame, by clicking here. He did a bang-up job on the presentation and crams a helluva lot of material into a 9-page write-up.
Looking at this setting it immediately struck me that it's pretty much the same size as the Frontier, the setting for TSR's Star Frontiers game. That's not a coincidence. There's other evidence that at least one member of the committee that designed Star Frontiers was cribbing from Trav. Why not borrow from the best, especially when creating a rival product? It's kinda funny how for years, decades now, people have been complaining that the Frontier was too small, too cramped. Yet here's a perfectly useable setting scaled on the exact same order of magnitude.
I think the big difference is that The Sky Aflame has real meaty conflict written into the setting, while the Frontier as presented in the boxed set is blander, with only the Sathar bogeyman and the corporate wars to give it any flavor. They did a lot more to spice up the Frontier in Zebulon's Guide, but that tends to get buried under overreaction to the new rules. (I'm just as guilty of this overreaction as anyone. I can barely read the Guide because the new rules annoy me so.) So it's not the size of your sci-fi campaign that really matters, it's what you do with it. Still, I can't help but think that I might prefer a campaign setting 2 subsectors in size (like the Island Clusters of Trillion Credit Squadron), or a quadrant (2 x 2 subsectors). But it's nice to know that a single subsector is quite sufficient for Traveller shenanigans.