That's a good-sized mata, but it's only half of the wild adult size. At my LFS there's a 1,200 gallon tank with a full-size yellowfin surgeonfish. It's almost the length of my arm (I'm not kidding!). I did not measure it but it's at least 20" long, and around 4" wide.
I hope people who put tangs in tiny-a$$ tanks can have the opportunity to see full-sized ones in person, then they'd understand why it's not a good idea to do that.