My heavy duty nest boxes are approx. 12 inches high x 6 inches deep x 6 inches wide and made from 20mm thick solid pine for long life.
They are perfect for blue, coal and marsh tits and are in accordance with RSPB recommendation the bottom of the entrance hole is 120 mm from the floor of the nest-box. If it's less, young birds might fall out or be scooped out by a cat.
My wife won a very elaborate nest box in a raffle that just fell to bits after a year. This inspired me to make a box that would last many years so birds can get used to it as they returned year after year. The boxes are available in a range of colours, blue, green, yellow, grey, brown and pink being inspired by the beach huts at Southwold in Suffolk and are reminiscent of many childhood holidays. They are made from thick red pine that I have salvaged from the building industry and treated with water based Cuprinol wood stain and preserver for long life.