While I respect Pete's experience and dedication to light weight, I disagree with him on tent sizing. I think the real capacity of any tent is one less than the manufacturer's stated capacity. So a 2-person tent is a good size for one person, but if you need a tent for 2, buy one made for 3.
Some of that may be because I'm tall and wide. :/ But there's other reasons, too; dressing, storage space for things you don't want to leave out (e.g. camera, wallet, glasses), reading, and just hiding out from mosquitos. I'd hate to be in a bivy sack on a warm night when mosquitos are biting through the tent, with no room for an air gap to my skin!