I have a 14x16 with 5' walls Davis wall tent and find it very comfortable and spacious for two people on a hunting trip. I have a small wood stove inside and set up a folding table to hold a bit of gear and supplies.
I would think that I could easily get three adults in it (but no more) and have plenty of moving about room (getting dressed and such)
For 4 to six folks I would definitely go larger... it isn't much fun to be crowded and taking turns moving around isn't an option in my mind.
Think about the size of a cot for each person and then the gear and clothing for each person. Sure, a fella can store clothing and belongings under their cot but when its time to get up or do something there must be room enough to have the gear out and taking up space. A table inside is really nice as are a few chairs (it rains outside sometimes). The stove takes up a fair amount of space too.
