Depends on purpose, people and personal preferences. It is not a simple question to answer.
Tent capacity is measured by the number of adult sleeping bags it will hold. This is standard sized sleeping bags, laid end to end or corner to corner on the floor of the tent. It does not allow for storing your gear, nor allow room to get out in the middle of the night without crawling over your tent mate.
A two man tent will generally be comfortable for one person plus gear, a four man tent will be comfy for two people plus gear and a six man will be comfy for three people plus gear etc.
Height is the another factor to take into account. Children may be happy to sit up in a tent but most adults will want to stretch out and stand to change their clothing. Look at the centre height of the tent to ensure you can comfortably stand up in it. This may not be a factor if you only do a couple of weekend camps twice a year but if you are travelling or taking longer holidays this could be highly relevant.
Your choice may also be influenced by the purpose and method of your travel. If you are backpacking, then light, fast, compact is critical and you may want something just big enough for you to crawl into with a sleeping bag and a backpack. If you are travelling by car then weight and size are not such a factor.
If you have to put it up by yourself, then look for something that is super simple and fast. Some larger geodesic domes really are amazing constructions but they are often two person job to erect. Ultimately, you want it up so you can start relaxing.
Take into account the shape of the tent you choose as this does impact on room. Dome’s will often have less space because they are oval in shape but nothing beats a dome in a strong wind. They are designed exactly for this purpose. They are super robust. A-Frame and geodesic shaped tents often have good height but you lose a little space because of the sloping sides and shape of the tent. This can be even more relevant if you are sleeping on stretchers vs. floor mats. A squarer (umbrella) shaped tent will always be more room efficient but they do not always stand up quite as well to strong wind and the height can make them a little harder to keep heated in winter. In heavy rains and high humidity you don’t want to be resting on the walls of a tent. Water will run off the tents but if something is resting against a tent wall or protruding out of it, it will likely get damp. Taking the next size up in a tent is often the safe option and will alleviate all these potential factors.
If you are struggling to visualise space... The average four man tent will be approximately 2.4 square metres and a queen size air bed will pretty much fill the entire floor space once blown up. If you are concerned about crawling over others, take into account the zippers for entry/exit. If these open and close on both sides of the tent it can make it much easier when getting in and out without disturbing others.
Choosing the right tent to meet your personal needs can be the difference between being sad your trip is over or being pleased you're going home. It is definitely better to err on the side of room and ultimately comfort if you are not restricted. More room is better than not enough.