The dimensions of your pop-up should match the needs of your adventure, that is to say, big enough for everyone who is sleeping inside of it in addition to a little extra room to account for people’s gear and personal belongings.
For instance, a 4 person group with their gear in tow will not fit into a 4 person tent, but might fit very well in a 6 man tent.
Similarly, a 2 person group would do better with a 3-4 person tent