The MS6 is a 15.9 gal tank. Right from the owners manual.
That may be true for the Mazda SPEED 6, but not for the non-performace oriented Mazda 6 which has an 18 gallon tank (for both 6 and 4 cylinder engines). See the following:
http://www.roadandtrack.com/default...Flash=1452&agMake=MAZDA&agModel=6&agYear=2008