From what I've read a 2x4 would probably be best. Gives them enough room to flatten their feet out so when they squat down, they keep their feet warm . So it helps to not get frostbite.
I have 2x4s wide side up for my girls and they all look comfortable, even the bantam. They shuffle forward so that their toes curl over the front of the roost while they sleep, and the rest of their foot is supported. Their toes are covered by their breast feathers, and in colder weather they sometimes shuffle back and sleep on their toes.