When deciding which tent to buy there are a number of factors to consider, to start there are two questions you can ask yourself.
1. How many people do I want my tent to accommodate?
The size of a tent is measured in the amount of people that can sleep in the tent (ie: a two man tent has adequate space for two adults to sleep comfortably)
2. Where am I going to be using my tent and what weather conditions can I expect in this area?
If you are using your tent for hiking or expeditions, you need to consider factors such as: weight, packed size, ease of set-up, uneven terrain, strong winds, snow and rain.
If you are using you tent in camp sites where you are pitching your tent near to your car then factors like weight and uneven terrain are not big issues and factors such as head and shoulder space and price are more important.