4x6x8 I am assuming that this is in feet. This cage is huge!!! Most of us are keeping boas in 4x2x12-16" cages or 6x2 cages. as a general rule of thumb, 1 square foot of cage floor space for every foot og boa. so with that in mind your cage could handle a 24' long Boa LOL, I have never seen one that big, but a Burmese python might fill the cage. Your cage is plenty big for a boa of any kind, but if a smaller boa is what you want, then there are a few that you can get.
Not even including all the Island Boas (Hoggs, Cay, Corn and so on) You can get pretty much any Central American Boa Nic, Honduran or even a male Colombian. (Males tend to stay smaller then females) most of these Boas will top out in the 6' range or so, but a lot will depend on feeding. If over fed a boa will over grow. Power feeding when young makes a boa grow to artifical size, so responsible feeding will help a lot.
Now as far as True Red tail Boas, The Ecudorian Boa (also sold as Amazon Basin Boa) tned to stay on the smaller size of around 7 feet and again the males would be smaller then the females. But other then that most True Redtails will get closer to the 9-10 foot size.
I hope this helps
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Jim Hopkins "Hoppy"
Hopkins Holesale Herps
Hopfam1@aol.com