The smaller the hole size the harder it is for the horse to consume enough hay/forage as they need...albeit slower.
I would not go any smaller a size for a horse to eat from than that net...
If anything, the mini would be given his net separate from the horse, as is the horse is separate from the mini...
AND, at some point my horse would be invited to eat loose hay free-will without the restriction of the net.
If you are restricting because of the mini... the mini might be restricted to a different area at free-feeding time.
If you are restricting the horse for a reason...then you need to do what you need to do, but be careful you still allow either animal enough to eat for their daily must have to thrive.
🐴... jmo...

 

My experience with using 1" hole nets for full size horses is not good. One horse got intensely frustrated. Another horse beat the nets so hard to get the hay out that she inhaled a bunch of dust and got a respiratory irritation. 1 1/2" is better. It kind of depends on your horses, so you're probably going to have to experiment. I have no experience with minis. Now I just feed them more frequently.

 

We use 1" hole nets all winter (purportedly - some are smaller/some larger than others, & I haven't checked with a ruler to see which are truly 1"). The horses get their afternoon and overnight hay in them; the morning hay is in Smartpak mesh 1.5" nets. All are tossed on the ground (barefoot horses, and the drawstrings have been untied so they're not loops, passed through toggle fasteners like a bigger double version of what you'd find on a hood drawstring, so there's nothing for the horses to step in).

They still manage to finish their nighttime hay before the night is half done - so it takes maybe 3x as long as without the nets, but it certainly doesn't keep hay in front of them all the time - nowhere near that. They do toss them around (and occasionally lose one against or under the fence) but they are not destructive to them. My mare has figured out that there are bigger holes around the drawstring and so she's not quite getting the 1"-hole experience until she finishes what she can reach that way. I will say they prefer the morning mesh nets but I think they go through hay quite a bit faster with those.

All of which is to say that 1"-hole hay nets work for us with two fairly easy-keeping full-size horses.

 

I use 1" holes. The idea is to slow them down - anything bigger than 1" and my horses can gobble up that hay in no time. My horses know they will get the hay out eventually, and they just work away at it. It takes them hours - but that's the point of a slow feeder net.

The best haynets I've found are the Nag Nets. Incredibly durable and the only ones that have no tears. Also the hardest to get hay out of. My horses just adapted. It's unlikely your horse will starve to death rather than keep at it until it gets some hay.

I have two horses and a pony. They all use the same nets.

 

Thank you everyone 😊!!
The mini is coming from a rescue where they free feed with the slow feeders...the openings they had looked larger, but, I don't think he was eating a lot at one time because he'd get run off and have to go to another feeder.
I told them I would continue to feed him with the slow feeder (he's used to it and I think it is a more natural way for them to eat)
The full size horse (Arabian mare) is getting a little chunky and had an episode of colic a couple of weeks ago, plus she's older so probably keeping her blood sugar level more consistent isn't a bad idea, so I think she would benefit as well.
I guess I can just get different sizes....I don't think the next size up would be too bad even for the mini, but, they'll be monitored....so, yeah, no one is going to starve

 

I have a Saddlebred hard keeper and a mini. The Saddlebred gets his alfalfa at night and the leftovers offered all day. He also has free access to good grass pasture. The mini only gets to graze on short lawn grass because in the pasture he selects the grass seed heads (grain) and gets even fatter. At night in his stall he gets orchard grass out of a 2" slow feed hay net and does just fine.