The short answer for a big, heavy hen
An adult Orpington needs about 4 square feet of indoor coop space per bird, 8 to 10 square feet of secure outdoor run per bird, and as much extra field or garden range as you can offer on top. Those figures are a floor, not a target, because an Orpington is one of the largest common laying breeds, with a deep body, dense feathering and a calm but space-taking way of moving around other birds. A coop sized exactly to a smaller breed's minimum will feel crowded for Orpingtons within weeks.
Why Orpingtons need room beyond the bare minimum
An adult Orpington hen carries considerably more weight than a typical layer, and that bulk changes how flock space actually works. Wide birds take up more roost length, turn less neatly in tight corners, and are slower to slip away when a flockmate gets pushy at the feeder. Crowding in an Orpington flock shows up as feather pecking along the back, hens reluctant to come down from the roost in the morning, and lower-ranking birds hovering near the door instead of feeding. Extra floor space is cheap insurance against all three, because this breed's placid temperament only holds when nobody is forced to compete for elbow room.
Ventilation matters too: heavy feathering traps heat in summer, so the coop needs high vents moving air above roost level, while winter bedding must stay deep and dry under broad feet.
Roosts, nest boxes and doorways sized for the breed
Give each adult Orpington roughly 10 to 12 inches of roost bar, and mount the bar itself wide and flat-topped rather than round and narrow, so heavy feet rest without curling hard overnight. Keep roosts lower than you would for a flighty light breed, around knee height, because Orpingtons are poor, reluctant fliers and a long drop from a high perch can injure a heavy hen's legs and feet. A short ramp up to the roost suits this breed far better than expecting a hop.
Nest boxes for an adult Orpington should be roomy, around 14 inches square, with one box for every four or five hens. A standard small box forces a big hen to wedge herself in, which cracks eggs and discourages laying in the box at all. The pop door deserves the same thinking: make it tall and wide enough that a broad, fully feathered bird passes through without ducking and squeezing, or subordinate hens will hesitate to go out and lose ranging time.
Field space and how Orpingtons use range
On open field, an adult Orpington is a steady, unhurried forager who works close to the coop rather than ranging to the far fence. That habit means run quality matters more than raw acreage for this breed: short grass, a dust-bathing patch of dry soil, shade through the middle of the day, and scattered cover such as low shrubs will be used daily, while a huge bare paddock will mostly sit empty beyond the first stretch. Rotate the ranging area if the ground near the coop turns to mud or bare earth, because heavy birds churn wet ground quickly and muddy feet track filth onto eggs and into bedding.
Watch-outs tied to space in this breed
Watch an Orpington flock for bumblefoot, the foot infection heavy hens are prone to, especially where birds jump down onto hard or stony ground; provide ramps and soft landing areas, and check the underside of feet when you handle a bird. Heat is the other space-linked risk: if hens are panting and holding their wings away from their bodies on a still afternoon, the coop or run needs more shade and airflow that day, not later. A hen who stops eating, isolates herself, or develops swelling on the foot pad needs a veterinarian's input rather than home guesswork.
Keeping the coop dimensions, roost plan and ranging rotation written down where the whole household can see them is easier with KinsLe Family Sharing, since the family members who feed, collect eggs and lock up the Orpingtons all work from the same shared details.