A 2x4, with the wide side facing up, will give the girls good support and plenty of space to get their feet tucked under their feathers.
I tried smaller sizes (2x2" and dowling) and those just sagged in the middle with the weight of the birds (our coop is 8' across. We're dipping back down to 0F for the second night in a row, and it's nice to not have to worry about the birds.

