Hello Samuel,

Yes~~ a 30 cm cube ( 12" cube) exo terra can be used for a QT enclosure or for a froglet grow-out for dart frogs.

I believe Lija has already helped you with this question.
As far as when (?) - if ever- you will need to change tank size.

Keep in mind please , I do not have a pac, nor have I ever owned one.
PLEASE refer to your previous post and question regarding tanks size

I will repeat what I have read here on FF .
A simple search using the search feature will find you 1000s of posts with information on this subject.

I believe females of some species can reach 8 inches. Lija would know exactly.
'She' could remain in a simple 10 gallon aquarium .
As always ---- be sure to maintain proper husbandry ( keep it clean !) especially the water !!!
"She" would never need anything larger than a 20. As Lija mentioned, wait ( " keep and eye on the frog's size" ) and see how large the he or she gets.

Certainly the proper size enclosure ( one that is not too big ) will be easier to maintain adequate temps and humidity.
Of course, it is also easy to keep to the typical bare bones set-up that is always recommended for them.

If you want to see something 'green' with your frog; buy a favorite plant and put in outside the enclosure / next to the tank.