40 mm = 1 5/8" mesh is the largest mesh that works for SlowFeeding. Larger than that and the horse can fill his mouth too easily and then he will not chew enough to feel content before he has become obese.

40 mm = 1 5/8" mesh does not restrict the amount of hay a horse can eat over a 24 hour period but it still makes the horse chew in a good manner and it minimizes the waste.

40mm mesh is 56% larger than 30 mm mesh and 31% larger than a 35 mm mesh.