If you're not planning to backpack or canoe camp, the size and weight of a tent doesn't matter so long as it fits in your vehicle. Tent capacity is based on the square footage and how many standard sleeping bags will fit in it. For example, a 2-person tent will accommodate just two people. There will be very little elbow room or extra storage space. You'll find a 4-person tent will be more comfortable for two people, and you will have space to spread out and store your gear too. For a family of four I recommend a 6-person tent. As a rule-of-thumb buy a tent that has a capacity rated two people higher than the number that will actually be using it.

You may want to check out the multi-room tents. If you're camping with the kids, a 2-room tent provides a little privacy. Multi-room tents come in 2-room styles, where the rooms are separated by an inside tent wall with a zippered door. There are 3-room styles that are like the 2-room ones but with an added screen room, which is nice for changing wet or dirty clothes before entering the other rooms, and which are great for setting up chairs or a table to use in case it rains. There are also 2-room tents, which have just one large sleeping area and an attached screen room. Tents with screen rooms attached are great for storing gear outside the sleeping area.

from http://camping.about.com/od/campinggear/a/tentbuyingtips.htm