A round entrance 1 1/8 inch in diameter will accommodate Chickadees and
exclude House Sparrows to the best of my knowledge. The sides of the
hole should be sanded smooth to prevent undue wear on the plumage. DO
NOT put a perch below the entrance. The inside and outside of the
entrance wall below the opening should be roughed up to ensure good
traction so the parents can light, and parents and fledglings can
negotiate an exit. Many commercially available "birdhouses" have
smooth and slippery interior surfaces that can entrap and kill
investigating birds. Good luck. Dont hesitate to contact me with
questions, comments, or observations.

Stan Kostka Arlington