It makes perfect sense that the aperture of our nesting boxes should vary. And that will depend on the guests we are wanting to attract. The diagram here, indicates an aperture of 32mm. There is nothing wrong with that at all and at that size you can expect to see a wider variety of birds checking out your box with the strongest most likely to prevail.
Blue-tits only require 1″ or 25mm to squeeze through as they are so tiny. They will however, nest in boxes with larger entrances up to 32mm but from a safety point of view, the bigger the aperture, the bigger the predator so do your homework on what’s best.