29 or bigger will work fine for most. Its not a long term thing so it doesn't have to be as big as your main tank. Make sure you QT tangs for at LEAST 6 weeks after seeing no parasites/infections, and always do a Hypo treatment with them.
If you want an entire article on how to do a really good QT (written by Lee Birch), send me a PM and I'll give you a link.