Inside dimensions must be large enough to accommodate the incubating biard and a brood of growing young. Entrance hole size is especially important. If the hole is too small, the bird you built the box for won't be able to get inside. If the hole is too big, predatory birds and mammals will be able to get inside and reach the nest, and cavity-nesting birds may not use the house. Use the proper materials.
Wood is the best material for birdhouses.
Never use creosote-treated wood as creosote may kill the eggs or chicks. Use galvanized nails, as these will not rust. Birdhouses need not be painted, but the box may last longer if you paint the outside. Never paint the inside of a birdhouse. Nest boxes that don't match the vegetation may be easier for predators to find.