The size of the entrance must be big enough for the bird but keep them safe from predators.
Most birds prefer an earth colored birdhouse.
Wood is generally best.
Bluebirds need small houses with 1.5" diameter holes. There should be a perch in front of the hole.
The entrance hole for a house wren birdhouse should be around 1.25" or less. The house should be placed 5 – 10 feet above the ground.
Birdhouses for chickadees should be in brushy areas, and around 4-8' above the ground.