That depends on whether you're a backpacker trying to conserve pack weight, or you're a family camper needing extra room for your kids.
A. Backpackers will generally compromise tent space for weight. A tent with enough room to unroll a sleeping bag is usually big enough for them.
Car campers, on the other hand, are not limited by weight so they can use tents that are bigger and roomier. Tent capacity is measured by the number of laid out adult sleeping bags it will hold. This does not include any elbow room. For family camping I recommend getting a tent with a capacity rated two higher than the number of campers that will use it. So for a family of two I would recommend a 4-person tent, for a family of four a 6-person tent, and so on. You can go even larger, if you want more room. If you're happy with how a tent packs, sets up, and stores, then size is of little consequence. Things to consider that might be more important than size are cost, ease of setting up, usability, durability, and functionality.
Depending on how you go camping should determine how big a tent you use. Backpackers will likely use the smallest. Car campers are free to use the largest. If you get to the campsite by boat, canoe, horseback, motorcycle, or some other method, you'll need to consider weight and packing size before floor space.
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