Smaller is better, there's less room for the wife to slide away
Our last camping trip the temps got into the low 30s, with a double wide r5 pad, a sheet on it for comfort, and both sleeping bags piled on like blankets it wasn't hard to stay warm even for her. I really don't know the truth on size mattering for temps but when camping off the Atlantic on a windy night with temps dipping into the 20s the 5 person tent didn't feel as cold waking up with me and 3 kids in it the same as the cabin tent in Montana on calm August nights even with it being all three kids, me and the wife. I've no problem believing it has no r-value, be hard to see how it would, but it also seems a tent would have less air movement and less surface area to speed up the transfer of heat vs a larger tent. I'd image a 2 vs 3 person tent might be more negligible, but my experience has been the larger the tent the colder is seems to get.