I think Daughter's greenhouse is about 12 x 8. Mandarins, finger limes, yuzu, keylime, kumquat... those are the ones I can think of. She keeps them in the greenhouse year round. I move things in and out but only have a couple of citrus left (gave the rest to Daughter). I think she is in zone 6-7 but the koi pond and one solid house wall (her's is a lean-to) keep it warm enough with just a milkhouse heater and a big fan to move the air. She also has cymbidiums and a few other terrestrial orchids.
Mine is free standing (12 x 16), my low temps are in the mid-20 but my 6 or 7 heat lamps keep things warm enough most of the time. If I'm concerned, I break out the milkhouse heater or the Buddy Burner. I'm wintering over citrus, cymbidiums, jade plants, birds nest ferns...
