Your poultry coop should be large enough to allow proper air circulation but small enough to keep from being too cold and drafty in winter. Adequate space for each bird is important for bird comfort and bird health. If you do not provide adequate space per bird, behavioral problems such as pecking and cannibalism may result.
The minimum amount of space per bird depends on the species, breed, and age of the birds and whether the birds have access to outdoors. The fact sheet Poultry Management Specifications (H.R. Wilson, F.B. Mather, and J.P. Jacob, University of Florida) provides information about minimum floor space, feeder space, and waterer space for chickens, turkeys, ducks, geese, pheasants, bobwhite quail, guinea fowl, ostriches, and emus. Keep in mind, however, that the specifications listed in the fact sheet are only minimum requirements; space requirements may be higher for some exhibition breeds.