The general rule is that you need to consider the capacity of the tent. This means you get a size that fits one or two more people than the actual number joining the camp.
This should provide space for the campsite you are bringing with you and match your personal camping legroom needs. You can also look for army tents via https://www.usmilitarytents.com/army-tents.aspx.
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When purchasing a camping tent, consider the following factors:
How long and wide is the tent? As a general rule of thumb, you need to anticipate the need for more space than you need. It's better to live in a tent that gives you more space than to store in a tent that forces everyone to spend the night in an enclosed space.
How high should the tent be? The same rule practices to the height of the tent roof – give a bonus. The idea is to make sure you can stand upright in the tent. If you get a tent so short that you have to crawl to get in, shake your head every time you get up in the morning.
Do the tents give you enough space? In particular, you need to make sure that your tent can accommodate your inflatable bed or sleeping bag should it blow up or roll-up. Otherwise, you will be left without the comfort of sleeping in a tent.