I have both of those species and here's my thought...

I have a trio of adult Leucs in a Exo Terra 18x18x24 and they have plenty of room. I believe even four Leucs would be comfortable in that size. I have ledges and horizontal branches and the frogs utilize every inch of the viv. I have a 20 gallon tall with a pair of D. azureus and even though I have branches and ledges they tend to stay mostly on the ground, although they do climb up to the heights now and then. I also have three female azureus that are about 10 months old in a twenty gallon long and they're doing fine.

I would go with the 18x18x24 because it would perfect for 3-4 Leucs or a pair of D. azureus.

I can't make a comment about the water feature because I have no experience with them. Personally, I've never wanted to bother with one because between the frogs, their food, and trying to maintain the foliage, all my time is all taken up.

Good luck!