[Tom Kirkham]

Yes there is a big difference. It is proportional to the surface area of the piston. So A=pi x dim x dim /4 Thus .75^2/.625^2 (note pi is in both the top and bottom and thus cancels out as well as the constant 4). So with the 3/4 in master cylinder you will have 1.44 times the travel over the 5/8 master cylinder. Note that the clutch pedal effort will go up by the same amount.