The thickness of the wood, which should be at least 15mm to provide insulation and to prevent warping.
It is better to nail your box together using galvanised nails to stop rust.
Drill a few holes in the base of your box to ensure that any rain that does get in can drain out.
Different size holes are suitable for different species:
25mm or larger for Blue, Coal and Marsh Tit
28mm or larger for Great Tit and Tree Sparrow
32mm for House Sparrow
More information about birds can be found on the RSPB website