MTN bike sizing

Looking for a ebay MTN bike to beat around on in the fall. What would be an avg size for me besides medium (i.e. 19", 17") I drive a felt 54" now. I am 5’9" with about a 30-31 inseam.

Would this work…
http://i3.ebayimg.com/05/i/000/a6/db/bfbc_12.JPG

why “besides medium”? sounds like medium (17") would be about right.

Those things are getting popular here among the Asian foreign exchange set, although I can’t imagine why. Then again, they’re the same group who drive around wearing welder’s masks…