General rule of thumb is to allow approximately one foot of roost space per bird - though some go with 9 inches per bird. As noted above, you do want the roost situated higher than the nest boxes to encourage appropriate roosting. 2x4's with the wide side up are a popular choice for roosts - if you are running a particularly long roost you may need to provide support(s) at various points of the length to prevent sway. I actually prefer a 2 x 3 for the roost (again, wide side up). Actual location of the roosts for your coop will depend on how your coop is designed -- is it a walk-in style coop, for example, or one of the shorter types? If you can post some photos that might help with suggestions.