30 mm = 1 3/16" mesh is the smallest I'd like to recommend because smaller mesh can be completely blocked by coarse strands or weeds.
Small meshes does slow the eating down but it does not solve all problems or make everything fantastic. To benefit in the best possible way from the marvelous SlowFeeding effects you must make sure your horse never runs out of hay. Choosing a smaller mesh that makes his serving last for 3 hours instead of 2 hours doesn't really do it as long as you don't want to go up and feed your horse a couple of times during the night. Perfect SlowFeeding is a delicate dynamic process that can not be executed by force or computers. The only way is to over time teach the horse to eat in a relaxed manner that makes him harmonic instead of even more frustrated and this is done by a combination of making and letting him chew enough to feel content without having to worry about running out af hay. It is super important that the horse stops eating of his own free will before he runs out of hay or hew will just it faster and more the next time.

The hungrier he is, the longer he has been starving (i.e. not been eating), when he starts to eat the easier it needs to be not to make him frustrated. When hunger is not his major concern he can accept that eating becomes more time consuming, like if the grass grow more sparse as long as he can not see the end of the supply and he is used to be allowed to eat (i.e. chew) until he is content. If his eating will be aborted, of any reason, before he feels content (i.e. has chewed enough to feel content) he will be frustrated and eat both faster and more the next time he gets the chance to eat.

Horses have obviously over millions of years adapted to an environment where there were less calories per surface area compared to maintained modern pastures and free choice hay and this is what we need to mimic with SlowFeeding or our horses becomes dangerously obese.