For board size, I prefer a 2x6 with the 6" side for them to sit on - this allows them to cover their feet well in the winter. As for how high up, make sure the roosts are higher than your nest boxes. If the nest boxes are higher, they will want to sleep in them at night. If you have multiple levels of roosts, they will try to roost on the highest one, and this may be a problem for the bigger girls when they try to fly off them the next morning. Even if you have a ramp they use to get up to the roosts, they will likely try to fly down in the morning and if they have to fly from too high, they risk an injury. I know what you're looking for is a specific height for each element but it really is variable according to your own coop and flock.
To give you a better idea, my roost is a 2-stage affair. The first stage is about 18" off the floor and is a 2x4. They jump from that up about another 18" to the main 2x6 roost. Once the higher roost is getting too crowded, a couple will usually roost on the 2x4. My nestboxes are also about 18" off the ground and are on the other side of the coop. I haven't had any issues with them trying to sleep there because they all want to be on the highest roost. I don't have a ramp up to mine because they don't have any problem jumping 18".
