It all depends on the species. I would say that you should have at least 18 inches of height for a more
terrestrial frog like an Arzureus. A more arboreal species like a leucomelas would defintitley need more height. And remember that a properly layered substrate would take up at least six inches
an 18x18x24 or 24x18x24 is usually sufficient. But again, for some species, the higher the better