There's also livability, the ways you'll most likely be using a tent, who you'll be traveling with and where you'll place it...
Consider is how much time you will likely be spending in the tent. If you tend to layover several days in storms, then it is well worth having some extra interior room to do so. If, however, you use the tent only for sleeping and are the type to get out and ride through bad weather (me), then you can get by with a considerably smaller tent. Other posters raise an equally important point -- will you often be bringing all your bags inside, or just your handlebar bag, leaving the rest on the bike as a kind of bureau chest? This will partly depend on where you are touring. I can leave my bags outside and exposed because I often camp in the wild/wilderness (and winch the food-containing bags high into a tree in Bear Country. The bike-mounted alarm wakens and alerts me if something is bothering the locked bike and its mounted gear). Around people, I'd feel far more comfortable with the bags inside to deter theft and reduce opportunities for pilferage. In that case, some extra room is very welcome. I can just fit everything but the bike inside, though it is crowded. For a night at a time, it is fine, but people vary in how much room they prefer or require. A few of my friends "size-up" by one person-rating when getting a tent -- two people often have more stuff than one in proportion to tent size, so the couples I know tend to favor 3-person tents to ensure they can bring all their bags inside as well. Tall friends often go up a size and sleep diagonally to get the length they require. You want to also watch the slope of the inner tent's roof and sides so there is a little room between them and your bag to prevent wetting-out from condensation. Ventilation is important to prevent condensation in the first place.
Though a delicate subject, a surprising number of people camping in colder conditions prefer to toilet inside using a jar ("pee bottle") as a receptacle for that purpose (label it to prevent accidental drinking by companions; I've seen it happen). That can require a bit more room to maneuver inside the tent, especially for couples and can be wakeful for both. I have always gotten out and away from the tent to do my business, and consider the view of the night sky to be an added bonus when camping. I've seen meteor showers and the eyes of wild animals I never would have seen otherwise. It is a great way to notice early changes in the weather, and has changed a few start times for me as a result, meaning I could pack and break camp in the dry instead of the wet.
A one-person tent provides me with some additional options for ledge camping, where there often is no room for something larger, and being solo means I can move my little domicile away from snoring companions. This is one reason why I bought extras -- if friends don't have tents of their own, I can take a spare, and we can each have the freedom to wiggle and thrash, snore, and adjust ventilation individually. Some folks are territorial and others tend to expand their holdings in the night till the other is squeezed into a corner. Not everyone sleeps the same way, and even an issue like "too hot" or "too cold" or "more" or "less" ventilation can make a difference toward <ahem> interpersonal harmony over the course of a tour.
Bottom line: Ya gotta sleep or things don't go well. Whatever it takes to accomplish that is well worthwhile.
I am scared to death to cook
anything in a tent, having spent some time in the healthcare field and seen the results of tent-fire mishaps. I cook outside, no matter the weather. Other folks feel far more comfortable with the idea, and think nothing of brewing a pot of tea or even a meal beneath the vestibule while warmly enconced in a warm sleeping bag inside the tent. To each his own, but cooking habits are better accommodated in some tents than others, so your own preferences are worth considering.
More and more (tunnel) tents are coming with side entries instead of end-entries. I lived for years with my end-entry bivy, but it got old, winching myself in and out inchworm-style on my elbows and heels and was tough when it was wet and muddy or snow was on the ground. The side-entry on my new tent is terrific -- I just roll in and out and can even go sitting-to-standing if I wish. Unfortunately, it is a different "handedness" than my sleeping bag's zipper, but is still convenient and rarely matters in the warmer months when I use the bag as an open quilt or duvet in the tent. A double entry can be a real convenience when camping with a companion; one person doesn't have to crawl over the other to enter or leave the tent.
Stakes vs. freestanding is another issue. Freestanding tents can be pitched on any surface, though stakes are needed to secure them in wind. Staked tents can require some extra care in choosing a pitch, and are not well-suited for hard surfaces like pavement or concrete. I can secure mine stakeless using my loaded panniers and the bike as deadmen, and also bring along titanium needle stakes for when the desert playa is just too hard for the wider alu stakes I need and use in damp, loose forest duff and pastures. Staked tents require a bit more forethought than freestanding, but it is still possible to use them in a variety of places. When I camped at parks in Canada years ago, one had to remember nails and a hammer -- the tent sites were usually wooden platforms! Closely related is the matter of lines. I prefer fewer to more, as they seem to tangle unless extra care is taken. On the other hand, lines add a lot of versatility. Rock in the way? Just shift that line a few centimeters either way and it is no longer a problem. I get by adding short little lines to my staking tabs. Sometimes you have to personalize a tent.
Finally, consider amenities. A small loop to hang a flashlight adds greatly to livability, though it is easy to add your own or simply use an LED headlight. Mesh wall cubbies or a gear loft help organize small items, prevent loss, and get them off the floor, increasing effective living space. I added glow-in-the dark zipper pulls to mine so I no longer fumble trying to remember where I parked the double-zip. I'd suggest trying the zippers on any tent you're considering. A rough-runner tends to get worse, even with the application of bar soap or lubricants, while smooth-running zippers seem less likely to snag and catch over time and are a bit more dirt-tolerant.
Best,
Dan.
