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I normally only use a 60 watt bulb in each of my coops, about 15 inches above the water bowl. It provides extended lighting, it keeps the water from freezing, for the most part, and it provides some warmth in the coop. My big coop has 2 floor, it's 4x8 by 6 ft high. They sleep/roost in the top floor and any heat from the bottom rises up. On those really cold blizzardy nights, I can leave the light on all night, and they still go upstairs to roosts, where just barely gets to. They have a whole in the floor with a ladder to get up there.
My smaller coop is the same size, but not 2 floors and not as tall. I normally put my older hens in there during the winter, and used to only use the light when it was really cold, but last winter, we put up a divider, making a smaller area for the food and water, so I can keep the light on more during the day, but it doesn't light up the whole coop, mainly keeps the water from freezing and provides a little warmth. It's rare that I leave that light on at night, and they do just fine.
I've never even had a frostibitten comb, not even from my 2 leghorns that usually spend the winter in the smaller coop.