This question asks about enclosure size specifically for the Rhode Island Red, and the answer is a coop-and-run system dimensioned around the breed's energy rather than its body alone. A Red is a medium-large hen, but she lives large: ranging widely, scratching constantly, and enforcing the pecking order with more vigor than placid breeds. A tank plays no part in keeping any chicken; what a Red needs is secure indoor roosting and laying space plus the largest practical outdoor area, arranged so a busy flock can spread out instead of colliding all day.
Where the size really matters: the run
For a Rhode Island Red, square footage outdoors outweighs square footage indoors by a wide margin. She will spend every available daylight hour working the run, and a run that is too small becomes bare, muddy, and socially tense within weeks, while a generous one with grass, shrubs or structures breaking it into zones, and a covered dry dust-bathing area absorbs her energy productively. If the run is fixed, plan it larger than you first think you need and surface the high-traffic strip by the coop door, because that is where a Red flock churns the ground to mud first. Multiple feeding and watering points set well apart are a size feature as much as area is, since a single station in a small pen lets the top hen own the flock's breakfast.
Coop dimensions and fittings for Reds
The coop itself needs floor room for the flock to move and preen on a winter morning before the door opens, roost bars with enough total length that every Red can sleep without touching a neighbor, and nest boxes at a ratio that prevents the laying queue and squabble that starts many a morning badly in this breed. Reds are hardy and tolerate a cool, well-ventilated coop happily, so spend the budget on space and ventilation rather than insulation or heating. Set roosts higher than the nest boxes to keep sleeping out of the laying area, and make the pop door wide enough that the morning exit, which in a Red flock is an eager rush, does not become a daily traffic jam that strips feathers and tempers.
Fencing and security dimensions
An enclosure for Reds must be complete in the third dimension too: a startled or determined Red flies better than her sturdy build suggests, so a run fence that merely contains heavy breeds may not contain her, and a netted or roofed top section over at least part of the run doubles as hawk protection for a breed that ranges far from cover. Ground-level security, mesh that keeps predators from digging in and a coop that latches shut at night, matters more than any measurement, because Reds roost deeply and are helpless once asleep. If the flock free-ranges beyond the run by day, the run still needs to be sized as the flock's full-time home, since weather, predator pressure, or your absence will confine them to it for days at a stretch.
Scaling from a pair to a laying flock
Reds should always be kept as a flock, and each added hen needs her share of roost, box access, and run rather than simply being absorbed into the same footprint, because this breed feels crowding socially before the pen looks full to a human eye. Feather-pecking, vent pecking, and one hen always last to the feeder are the enclosure telling you it is undersized for Reds specifically, even if the same pen kept a calmer breed contentedly. Enlarge the run, add a second station for everything, and add perches and barriers before considering any other intervention. KinsLe Family Sharing keeps the enclosure layout and flock notes on the Reds' shared profile, so everyone can see the current setup and the changes that settled the flock.