I would try going without the light if you can in such a small space, it could cause drastic temperature swings. If you are concerned about them being cold, you could wrap the coop in insulation and throw a tarp over the insulation. But make sure they still have ventilation. That's what I did last year with my coop that was about that size. Also put a thermometer in there that you can read from the outside (window or remote sensor). I think you would be surprised at how much heat those hens will put off, and you will know if you really need to supplement the heat.