Well currently I have a 36X18X18, which is roughly a 40 gallon and from what I've read should be fine for the rest of my beep's life. But I think you can actually go smaller than a 40 gal., plus the height on my tank is probably not necessary at all. I have a lot of clutter and some extra hiding spots in the tank right now because he isn't full size yet.
Im sure somebody with more experience will chime in, as I'm not anywhere near an "expert".
EDIT: in all honesty, you might want to go with something shorter in height. And also, I know this sounds hypocritical but it may be best to just upgrade the tank size as you go, so your beep doesn't feel insecure. If I wasn't on a budget I would've done that myself, so to substitute I added extra stuff for him to hide in (as I said above).
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