My house is very cold and I used a 250 watt bulb with no problems. I can get a lower wattage bulb as they grow older, but they only come in white and I don't like having a bright light on all the time. My brooder is pretty big, and the chicks usually start by huddling under the lamp until they are about a week old.
If your chicks are huddling under the lamp---they are to cold. Just for info you can get black light bulbs for sure in the 100 watt because I got some. Just curious---how high is your 250 watt bulb from the floor the chicks are standing on?