There is a myth floating around on this forum that chickens' feet are not like all other birds' feet and cannot grasp around a tree limb or something round like that but instead their toes stay perfectly straight when they perch or roost. It's usually best to not believe everything you read on the internet.
There is a thought that if you live in a truly cold climate, chickens roosting on the flat side of a 2x4 will settle down on their feet and keep them warm. I use tree limbs of varying sizes and have never had a problem with frost-bitten feet. It seldom gets much below zero Fahrenheit here so I don't have experience with truly cold temperatures, but the flat board theory has a certain ring of logic to it.
Some people have reported on here that changing the roosts have caused chickens to start roosting or such, so they believe that chickens care about roost shape. There may be something to that. I really don't know. I have tried putting boards up with my roosts to see what happens. My chickens pereferred the round tree limbs, but chickens are creatures of habit and maybe they just did not like the change. I've also tried changing the tree limbs around to see what happens, since my tree limbs are not the same diameter all the way across and they are not straight, so the high spots change. Some of them did change their roosting position when I did that, but I think it had more to do with the new high spots than the diameter. The position of the window probably had something to do with which chickens switched and which did not.
I really don't think it matters that much what shape they are. Different things work for different ones of us. I'll repeat a few things that others have said. Use wood. Plastic and metal are too slick and/or cold in the winter. If you use flat boards, round off the corners to make it easier on their feet and to remove splinters. I'd suggest sanding them. If you use wide boards watch to see if you need to scrape the poop off of them. Chickens should not be roosting in a layer of poop.
Here is what mine look like when I am down to one tree limb with my basic laying/breeding flock in their favorite roosting spots this past spring.