I've done a lot of camping (no full-on backpacking, though, but lots of 5-6 days at a time) but just did my first loaded bike camping mini-tour last year. I just used the tent I already had, which is REI's 3-person Quarter Dome Ultralight (a few-years'-old version thereof; the new ones are a bit different.)

The trip went well, and I'm looking forward to more tours. I started thinking, well, I *have* to get a smaller tent! At least a 2-person instead of a 3-person! But then I started researching, and very quickly realized that in most cases, the weight difference between 2- and 3-person tents of the same manufacturer is very minimal. (I wouldn't consider a single-person tent; the general rule of thumb of *all* tents is that the true human capacity of a given tent is "what the manufacturer says - 1." ) If I were backpacking, I'd probably consider the weight differential more crucial; on the bike, though, I don't think I'd even notice it.

Volume, though, is an issue on the bike. Even there, the difference would not have been significant unless I went from a 3-person to a single. But then I lit upon what I hope will be the solution:

http://www.rei.com/product/766675

I just got two of these the other day: one for the tent, one for the rainfly. They compress each down to something that's about bigger than a softball, but smaller than a child's basketball. (For my tour, I had a larger-size, more expensive version that not only compresses, but is a dry bag -- cool! -- for my sleeping bag, which had to ride outside of my waterproof panniers.)

Now, if you're willing to go all hard-core with just a teeny bivy sack or some mosquito netting over your face, that's a whole 'nother level, and I don't have any experience or opinions on that!