35 mm = 1 3/8" mesh is SlowFeeding standard. It works with all hay qualities and for all horses regardless of age, sex, breed and activity level as long as the horse is used to SlowFeeding and the supply of hay is endless. It is very important to understand that a horse is programmed to eat until he feels content and if he is not allowed he will be frustrated. Using a mesh size that is to small and he might not be able to retrieve enough hay to chew himself content even though he will be standing at the hay continuously. Choosing a too large mesh that will allow him to fill his mouth too easily and he will not chew each strand enough and therefor eat too many calories before becoming content. All it takes is to stop the horse from filling his mouth and make him forget that there ever will be an end to the supply of hay and he will eat and chew in a relaxed matter which will make him content. Since soft hay is much easier to retrieve through the mesh so the mesh size must be chosen accordingly and the opposite goes for coarse hay that can be considerably more difficult to retrieve through the mesh and therefor might go better with a larger mesh if the horse is not perfectly accustomed to SlowFeeding then the horses seems to eat naturally anyway.