A snake that size will have a fairly deep and wide enclosure because it needs a good bit of floor space. It's easier to reach into the back corners of a 24" or 30" deep enclosure if it's at least 18" high.
Also boas will climb and use vertical space if you them something to perch on. I would set up a 9 foot boa in either a 6'x2'x18" or 4'x30"x18" enclosure.
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