I think if you are changing to a dual MC, you may want a bit smaller diameter than the original, because you are not pushing to all 4 wheels with one chamber, but splitting them. As Mike B stated though, if you use one that is the same diameter as original, it should push enough fluid but, may be too hard to push. Smaller diameter, makes it easier to push but, the pedal will travel farther to push the same amount of fluid so, if you go too small, it will hit the floor before stopping you!
