There is a difference in what they need and what is convenient for you or them. They don't need much, but there is nothing wrong with making it easier for them or for you.
The height is the easiest answer. Make them higher than anything you don't want them to roost on. How much higher? I hate hard and fast number answers. As soon as you give one, somebody comes up with an exception. The chickens need to see the roosts as the highest thing available for them to roost on. Maybe that is 6", maybe a foot higher than anything else? I suggest you keep them as low as you can while still making them clearly higher than anything else. This is just my opinion, but I think there is a lot of unnecessary worry about chickens, especially the heavy breeds, hurting themselves when they jump down from high perches, but animals do occasionally hurt themselves. I don't know of any benefit to having an extremely high roost. Besides, one of the best times to catch a chicken for treatment, inspection, or marking is when they are on the roost. Make it easy on yourself.
Do they need a ramp? If you have Silkies or something that can't fly, yes they will need a ramp or a sloped ladder they can hop up. Mine have no problem jumping up to my roosts, which are 4 feet off the floor. Mine are full sized chickens, not bantams or the supersized Jersey Giants. I'm sure mine could get to roosts a lot higher if they needed to, but a lot of them choose to use the top of the nests as an intermediate step. From watching them get up the 4 feet, I believe most of that wing flapping is for balance more than actual flying. So whether you need a ramp or ladder of not depends on your unique circumstances, but giving them the option does not hurt, even when they don't "need" one.
What size do you need? I use tree branches that go from maybe 4" diameter to 1-1/2" diameter. The chickens don't seem to care what diameter they are. They roost on all different diameters, wherever they feel like. There is a theory that in cold weather, it helps to have a big flat perch so the chickens can cover their feet with their feathers when they roost on cold nights. I don't live where it gets that cold, seldom much below zero Fahrenheit, and when I see mine squatted down on the small sections of my tree branch roosts, I can't see their feet because of their feathers, but maybe there is something to that theory. I can't say that theory is bunk, but in my experience, it is not necessary. If I lived where it gets to -30 at night, I might feel differently.
Do you need to trim the roosts? If you use boards, I would. It is not so much for the comfort of the chicken's feet as they curl around the edges, but I'd sand then to round off the edges to remove splinters.
How much roost space do you need? As much as you can reasonable provide. When they roost, they don't take up much space on the roosts, but they need space to spread their wings get to the roosts. Then when they get to the roosts, they will move around and position themselves so they can sleep in the positions their rank in the pecking order requires. The need room to maneuver. I've seen them knock each other off the roosts doing this. This includes being knocked off the wide portions of my roosts and not just the smaller sections, by the way.
But a big reason for extra room for me is that I integrate young chickens to my flock. Because they are young, they are at the bottom of the pecking order. Sometimes older hens are so brutal to the young ones that they leave the roosts and look for a safer place to sleep. The clearest example of this is where a broody hen has taught her chicks to sleep on the roosts, but when she weans them and they are on their own, a different hen goes out of her way to be brutal to them on the roosts. I've had these chicks leave the roosts and sleep on or in the nest boxes or even start to sleep outside the coop. I put up extra roosts to give these chicks room to get away from the bullies.
This kind of stuff does not happen each and every time and your circumstances may be different than mine. I'm sure others have had different experiences. If you have a lot of chickens, the average space needed per chicken drops because they don't take up much space once they settle down. So I just suggest as much as you reasonably can and see what happens.