How big does a blue tit nesting box obligation to be contained by diameter and how big the entrance hole.? Hey hey, keep it verbs! I put up a blue tit box which...

Hey hey, keep it verbs!
I put up a blue tit box which is cylindrical a couple of years ago and although much interest there have been no sign of nesting. Was wondering, I don`t know too big or entrance hole too big/small. I have learn that it must face north (awaiting compass to check) Any counsel welcome (no porno idiots please!)