Jamie's has one of those 4x4x8 juvenile arboreal enclosures which should be perfect for a larger sling or juvenile.
Currently my C versicolor is in an ExoTerra nano. I put him in there when his legspan was about the size of a silver dollar. He adapted very well.
Avics need absolutely NO extra humidity beyond the waterdish, which should be elevated. Some may not be willing to go onto the ground to drink. YMMV.
They can get mold infections in their book lungs and die suddenly if the humidity is beyond that of the average home, but keep that water dish full at all times. Do NOT overflow it!!
Once they create a web, just drop feeders into it. This makes enclosure size less critical since their web is in a sense the true size of their enclosure.
Give them lots of elevated webbing anchor points where you want them to go, not just a bare featureless enclosure.