Its not just the size...they're very active swimmers. Mine is now in my 240 and he is always swimming back and forth over the whole tank. He's probably about 4". When were you going to upgrade?
Which "powder brown" are you looking at? Is there a small white patch under his eye, or is it a much larger patch, going almost all the way down to his mouth? If the former, its Acanthurus nigricans, which is supposed to be very difficult to keep. If the latter, its Acanthurus japonicus, which is what I have, and should fare a bit better in captivity.
jds