Here at LowerGear, we get that question a lot, and I've got to say, we like it. That means you won't find out the hard way that you rented the wrong size tent!
What size rental tent you need really comes down to where you're going, how many people are sharing, and how cozy do those sharing the tent want to be. And what you should rent for a multi-day backpacking trip is a lot different that what you need car-camping with the family. The expected weather can play a role as well, although most of the tents we offer are equally protective for "3-season" use - for anything but heavy snow or high-alpine winter winds. See our full selection of tents here, including some designed for really harsh weather.
Tent manufacturers label their sizes in terms of "persons", i.e. 1-person tent, 3-person tent, 12-person tent (really!). So if you have a four-member family that primarily goes car camping, you can start looking at the 4/5/6- person tents right away. If you're more of the wilderness-loner type, a 1-2 person tent would probably work just fine. Keep in mind, if you buy a 3-person tent for three people, it won't leave much room for any other gear. The general rule is that if you want a little extra space, rent at least the next size bigger. Meaning, those same three people that want a little space, should consider looking at a 4-person tent or larger. Also, don't forget to check the manufacturer's dimensions - you'll find most such specs in our product descriptions. You'd hate to get into a tent that's shorter than you are, but generally if you're under 6' 5" you won't have many issues except for some small backpacking tents.
I've mentioned twice now, "car camping". Briefly, car camping simply means that you're able to drive up to your campsite; whereas, "backpacking" typically means that you have to carry all of your gear you could ever possibly want on your back. With that in mind, weight could be an important factor too. You wouldn't think a six pound tent would be heavy until you've carried it for eight hours a day for five days straight. Then you know. And not every tent use is in the wilderness; we rent to a lot of music-festival-goers as well.
Learn a lot more about selecting a tent on our Tent and Camp Setup Advice Page.