The rule that matters most: higher than the nest boxes
Hens instinctively roost at the highest sleeping spot they can reach, a leftover of roosting in trees out of reach of predators. That instinct is what you are designing around. If the nest boxes sit higher than the perches, the flock will sleep in the boxes — fouling the bedding, dirtying eggs, and encouraging hens to go broody in them. So set the roosts clearly above the boxes, and if your coop layout puts boxes and perches at the same level, lower the boxes rather than raising the perches sky-high. One level of roost is better than two for most backyard flocks, because a single height ends the nightly squabble over who gets the top bar.
Match the height to the birds you keep
For most laying hens, a roost somewhere around knee-to-waist height — roughly one and a half to three feet off the floor — suits: high enough to feel safe and clear the boxes, low enough to hop down from without harm. Heavy breeds such as big Orpingtons and Brahmas are the exception; they are poor fliers and land heavily, and repeated jumps from a high perch onto a hard floor batter their feet and legs, so keep their roosts low, around a foot to eighteen inches, with a soft landing of deep litter. Light, flighty breeds will happily use three or four feet and resent being low. Bantams fall somewhere between, and a mixed flock does best with a mid-height roost plus a ramp or a couple of step perches so the heavy girls can walk up rather than jump.
Width, space and what is underneath
Height is only half the perch. Each hen needs roughly a foot of roost space in warm weather — they spread out in summer and huddle in winter, so a little extra length beats a tight fit — and the perch itself should be a couple of inches wide with rounded edges, like a smoothed length of timber, so feet rest flat rather than gripping a narrow dowel all night. Keep roosts away from walls enough that tails are not pressed against them, and think about what is below: droppings fall straight down from a roost, so a droppings board or your deepest litter should sit under the perches, and feeders and drinkers should never be placed where the night's droppings rain into them.
Watch how they use it, then adjust once
Fit the roost, then let the hens mark your work. If they are sleeping in the nest boxes, the roost is not high enough, not comfortable enough, or the boxes are too tempting — block the boxes at night for a week while they learn. If a heavy hen is sleeping on the floor, the jump is too much: add the ramp or drop the bar. If one hen is always on the floor while others roost, check for bullying at roosting time before blaming the carpentry. Small adjustments in the first fortnight are normal; hens are creatures of habit and will use a well-placed roost for years once it feels right. Tape measure in hand inside the coop, still unsure what suits your mix of breeds, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.