I think they are a good starter arboreal but I am rather biased. Nothing wrong with starting with a GTP or ETB if that is what you really want and are prepared for.
Adult ATBs get about 4-6', keep in mind they are very slender and have very long, thin tails. The widest part of the largest ones I've seen are still more slender than, say, a can of redbull.
I didn't have many photos holding them to show size but here's a few that might help give you an idea:
baby:
this girl is a little bigger now, maybe a foot longer and a bit thicker than in this old photo but it gives you an idea:
I always like the way they will coil and eat while suspended
hanging out where the branches cross
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