This comes down to many factors, but similar to the horses. Despite having no top teeth, we have had cattle eat from 3cm nets. We recommend 4cm however and if the hay is course and stalky and not as palatable, then 6cm would be better.
Donkeys fall under the same rules as horses:
* If this is their first time use of a slow feed hay net, then we recommend 4cm. If your Donkey has reached 'expert' level status at eating through a 4cm, then come down to a 3cm.
* More palatable hays require smaller holes, such as lucerne or rich pastures.
If your hay is stalky and not so palatable, then 6cm would be great, but watch hoof size V's netting size and always ensure the net hole size is smaller than the actual Donkeys feet.
Alpacas and Goats cope well with both 3cm and 4cm nets, so it's really a matter of palatability of the hay and stalkiness.
You may have to experiment with different sized holes. We have found with our own horses that hay they may eat in summer in a 4cm hole, they won't eat unless in a 6cm holed net in winter.
Be aware too that they will eat through the hay quicker in winter than summer due to needing more fibre in their bellies to keep them warm.