The general rule is that you should give an allowance for the tent's capacity. That means getting a size that's labeled to accommodate a person or two more than the actual number of people that's joining the camping trip. This is to give way for all the camping gear that you will be bringing along, as well as satisfy the campers' personal need. You can easily find suppliers of military tents online.
Here are a couple of factors that you need to consider when buying camping tents:
What is the length and width of the tent? As the general rule mentions, you have to anticipate the need for more room than you actually need. It's better to spend on a tent that gives you more space than to save on a tent that will force everyone to spend the night cramped in a small space.
How high should the tent be? The same rule applies for the tent's ceiling height – give an allowance. The idea is to ensure that you're able to stand up straight while inside the tent. If you get a tent that's so short that you have to crawl just to get inside, then expect to bump your head on something each time you get up in the morning.