It's nice to see you on the forum, Holly.
I have never used the Exo-Tera terrariums, but other people seem to like them. The general rule we follow for keeping dart frogs is 1 frog per 5 gallons (if one were converting an aquarium) with a minimum size of 10 gallons. So the small 12X12X12 would hold approximately 7.5 gallons (rounding off roughly.) 12X12X18=11.25 gallons. 18X18X28= 25.5 gallons. 18X18X24=24 gallons, and 24X18X24= 45 gallons.
Then one must consider what one hobbyist refers to as the "foot print" or ground space preferred by different species. Some of the larger, less athletic frogs that tend to stay more on the ground than climb, like a larger "foot print" while others, usually the smaller thumbnails that climb and may tend to stay in the upper branches and background foliage require somewhat less. This is all rather arbitrary, because in a confined space, up and down is a very small difference compared to a rain forest. In my own experience, bigger is usually better.
Some dart frogs get along quite well in groups, while others can be extremely territorial, and for best results, should be kept alone or in mated pairs. With most species, it's the females that cause the most problems with territory.
So search around and find which ones appeal to you, then go from there. The breeder can make further suggestions. Those I find do well (or even better) in groups are the D. leucomelas, D. galactonotus, D. auratus, D. imitator, P. terribilis, P. vittatus, and P. lugubris, while the pumilios, reticulatus, azureus and tinctorius are generally territorial, to mention just a few.
One of the most useful books, although it is by no means complete, is the Professional Breeders Series POISON FROGS (W. Schmidt & F. W. Henkel.) A more complete volume that many of us have been anticipating is POISON FROGS Biology, Species & Captive Husbandry, which was supposed to have been published in 2005, but we've yet to see it in the U.S. by the same authors. It's going to be quite expensive, around $135.00.
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Patty
Pahsimeroi, Idaho
D. auratus blue
D. galactonotus pumpkin orange splash back
D. imitator
D. leucomelas
D. pumilio Bastimentos
D. fantasticus
P. terribilis mint and organe
D. reticulatus
D. castaneoticus
D. azureus
P vittatus
P. lugubris
