https://bushfarms.com/beeshardwarecloth.htm
People are often talking about various sizes of hardware cloth. Here is a list of the sizes and uses in beekeeping:
The #x is how many wires there are to an inch of hardware cloth. So #4 has four wires to the inch. Sometimes called 1/4" hardware coth. The openings are the # minus the thickness of the wire.
What hardware cloth to use
#8 - no bee can pass. pollen gets stuck but you can sometimes shake some through.
#7 - as long as there are no bent wires, no bee can pass. Pollen falls through. Good for the bottom of a pollen trap to let the pollen fall into the tray.
#6 - worker bees can squeeze through (and will) but they have to wiggle and squirm a lot. Drones and queens cannot.
#5 - worker bees can squeeze through easily but lose some of their pollen. Queens and drones cannot. Good for a pollen trap to force the workers through.
#4 - all bees can pass (workers, drones and queens). The workers sometimes lose some of their pollen when they catch their basket on the wire. (mice cannot and this makes a nice mouse guard)