Real SlowFeeding makes the horse feel better and makes the hay last for longer.
RB Std Low B
RB Std Low B has a mesh size of 3.5cm = 1 1/2" that is standard and always recommended for all kinds of hay and all kinds of horses. It is designed to cover a standard round bale from the top down. It fits tight on a 120cm = 4' round bale. This solution is only recommended when used together with plastic feed rings because if not there is a well-known issue with horses spending all their free time trying to get their heads in under the net.
The actual height of this net is 145 cm = 57" and the diameter is 170 cm = 67" just to make it easier to dress the bale. I recommend you not to consider this net for a bale larger than standard.