At Lucky Glider Rescue & Sanctuary we have over thirty cages between pets and rescues. At any given time, depending on the level of adoptions, there are between 75 and 95 gliders here. Three of the cages are giant "monkey-proof" cages 6ft tall that can fit three people. Two of these are occupied by larger pet colonies and one is a "hotel" for the rescues. Three or four cages are "Frankencages" that we created by chopping-up and re-assembling smaller ones. There are also a few home-made stainless steel ones that have been donated that people can literally fit into.
We have observed; however, that during the day anyway, cage size does not matter much. They are just louging or sleeping all huddled in their teeny weeny nesting box or pouch. However; at night when they come out to eat, they are scampering all over the cages and beg to get the heck OUT! At that point, I would argue that give or take a few feet, most cages are too small anyway and we are all fooling ourselves by arguing over the size.
Regardless of cage size, what seems to keep them happy is the following:
1. Variety. Gliders seem to like a change in venue every once and a while. Because most of the animals we care for are in smaller cages, we rotate them in and out of the giant cages at least once a week so everyone gets a chance to spread out and run and jump a little.
Clearly, not everyone can afford a giant monkey cage, and clealy, not everyone has the ROOM to install one. So if you do not have a giant cage, you can let them have the run of a glider-proof bathroom all night long like once or twice a week. Even swithing them between different shaped cages back and forth is a good idea. (You should consider a spare cage anyway in case you need to separate your gliders for health or aggression reasons).
2. Out-Of-Cage interaction. Even if you have a small cage, getting them out of the cage to spend an hour or two with you every day works wonders for their attitude. In fact if you carry them around a lot and play with them plenty outside of the cage, you should not worry too much about the size of the cage.
Clearly, these animals hate being locked-up. But we have observed that it's the out-of-cage interaction that really counts more than cage size. In other words, we believe it's more humane to spend time with them out of the (small) cage and truly treat them like one of the family than it is to ignore them even if they have a big cage.