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10g is generally accepted as a good size for a QT tank. Larger is better but for a fish that size, 10g should be OK. Put a few PVC fittings in it for the fish to hide in and he should be OK.
I guess it all depends on the fish, larger fish would need a larger QT setup. Tang's IMO would get stressed out in a 10G especially a 4" one. I use a 40G breeder as my QT. I would recommend you go as big as you can afford.