We were asked about the nest box entrance hole sizes and this was our reply:

Most have 30mm but box 5 has 25mm. Some started with smaller holes but have been enlarged by occupants or a woodpecker. Some now have aluminium plates under the outer wood front. Box 7 has a 40mm thick, wooden front. Box 5 (25mm) has only been entered by blue tits although great tits have been seen at the entrance. Box 4 has 30mm but it has only ever had interest from blue tits. The others have been visited by both blue & great tits.