I actually have my 3 adults in the ZooMed 18x18x24, and they certainly seem to have plenty of space. They all get along great in there, as well.
I think it's a good size for 3, but if you add a 4th, it wouldn't be large enough.
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Owned by the following;
1 male minilop rabbit
1 male shetland sheepdog
1 male maine coon mix cat
1 female winter white dwarf hamster
1.1 corn snakes
3.0 white's tree frogs
0.1 Leptopelis vermiculatus
1.3 crested geckos
1.4 leopard geckos
1 male crowntail betta
1 male veiled betta