Rockink,
I believe it is 1/4 inch. I made 2 of them about 8 years ago. The idea is to make it so that only one bee can travel in the slots and they cannot turn around. Mine was in the shape of a equilateral triangle with the entrance at the center. If you have SHBs, I recommend that you do not waste your time. It takes to long for them to work and then by the beetle larvae are hatching and slime the honey.
You can buy a plastics escape that fits in the slot of the inner cover. I use them for Trapouts.
Jim