MTGarden Gal, I'm not really the best person to comment. I have never had a greenhouse smaller than about 5' by 16'. Of course, I see the advertisements for greenhouses that are the type you are thinking of.
I often set up small tunnels of about 4' by 8'. But, those are very low and with a lot of exposure to the soil/lawn.
With an exhaust fan running and a vent open in the opposite wall, the temperature in my current greenhouse (180 square feet) was up to 75Ã‚Â° with 53Ã‚Â° fast-moving air, outdoors. Without the exhaust fan and just 2 vents open, it would probably been closer to 85Ã‚Â°. It could have been more open to the outdoors however, if I'd open the door. With so much wind, that would allow too much cool air in, however.
One of very small size will probably both heat and cool quickly.
We are each other's harvest; we are each other's business; we are each other's magnitude and bond. ~ Gwendolyn Brooks