I did some searching, and the smallest I could find was 60 gallons, but most sites say 100 to 125 gallons. They need a much larger water volume because they are so sensitive to water parameters and because other fish stress them out so much. I think putting a boxfish in with a clown or two would also be very stressful, as clowns are typically very aggressive.
There is an LFS in town that kept a pair of baby boxfish in a 29 gallon tank. The fish were about the size around of nickels, though, and he said even that was super risky. He did not keep any other fish in the tank and once they grew, he moved them into a larger display tank.