22 lbs

Recommended for no more than 22 lbs of dry loose hay.

This is the smallest size that can work for Real Slow Feeding. Since Real Slow Feeding means that your horse must have a seamless supply of hay these nets probably needs to be filled both in the morning and in the evening. 
All nets have a max capacity of almost twice of what is recommended but that would take at least 4 times as long and that would be wasting your precious time every time you fill the net and that it can never be worth.