[DanEC]

It would affect travel quite a bit. The area of a 3/4 cylinder is .4416 square inches. The area of a 5/8 cylinder is .3066 square inches - or 69.44% of the larger one. So basically for the same volume displacement of hydraulic fluid to the slave cylinder, the 5/8 inch cylinder will have to be stroked 1.44 times further that the smaller one. And then because of the multiplier effect of the clutch pedal arm ratio you have to push the pedal several times that.

For simplicity sake say you have a 3/4 inch slave cylinder and most have said you need at least .9 inches of slave movement to fully release the clutch. So a 3/4 inch master would have to stroke .9 inches also. With a 5/8 inch master you will have to stroke it 1.296 inches to displace the slave by .9 inches. A pedal ratio of 5:1 sticks in my mind for most clutch pedal set ups so that extra .396 inches MC movement would work out at 2 inches more travel at the pedal face.