I feel like for an adult a 24 cube is fine. Green trees stretch out, not up. The taller the cage the less likely for the chondro to go to the bottom and drink. If not properly misted it can cause major problems with dehydration. I keep neos in 6.5 and 12Q tubs and adults get 24L 18H 17.5 deep tubs. (goofy sizes huh?)
I plan to be moving up to 24 cubes or similar sized tubs soon. Screen tops are super hard to pull off with these guys. If you use google you will find the fix for that though. Pretty simple and really cool looking. The one advantage plastic display cages have over glass besides the the obvious (heat, humidity) if no water spots!
Here is the flip side. I have read debate that green trees COULD use bigger tubs as babies and that would make them more active in adult tubs. It is thought (possibly) we train them to just perch all their captive life. From personal experience wild caught chondros don't cruise any more than CBB ones.