I believe at least one person may have misread the o.p., as it is 8 sq ft for just TWO hens (both bantams), which is perfectly respectable space.
First I would suggest insulating, perhaps a *lot*, and include a good depth of bedding they can snuggle down into (this works best with shavings rather than straw or other things).
Second, as you have only two small hens in a decent bit of space, I would suggest keeping the coop very clean by removing any visible poo daily (easier if you can put a droppings board under the roost), and seeing how little ventilation you can 'get away with'. Yes, this is me saying that
- low stocking density in a very clean dry coop does not necessarily *require* much ventilation.
Third, once you have put in ample insulation (including the ceiling) and all that, you may find that a 40-60w bulb *does* make a significant difference now. In the (IMHO pretty unlikely, but who knows, esp if you are in the mts and it's a cold winter) event you should decide you need more than that, I think would suggest doing whatever it takes to stay AWAY FROM high wattage bulbs. No heat lamp in such a small low space, for a variety of reasons. There are a variety of options available, I have to go feed the kids lunch now so can't go into details but basically it's things like a)
several low-wattage bulbs, b) a low wattage panel-type heater, c) a mat type heater, etc.
Insulate, bed deeply and see how little ventilation you can get away with, though, and you may find they are fine
Good luck, have fun,
Pat