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It's not clear what year your bike is and what the original equipment would be.
For a single disc Ducati use a 13mm master (12.7 actually or 1/2") this moves enough fluid for 4 pots and gives sufficient leverage. For twin discs they used 15mm with the coffin type and then 16mm for the Goldline remote res ones. Goldlines would have come in about 1999.
A smaller master piston will apply more lever force to the caliper but will need more lever travel. a larger master piston will feel a bit sharper but need more finger pressure for the same braking force, it will have less lever movement.
When I had a 750SS with single disc and 12.7mm coffin master, I upgraded to twin discs, but retained the 12.7mm master. This resulted in a slightly larger lever travel but the force that could be progressively fed into those brakes was phenomenal. The feel and feedback was very good indeed. Power + control = the holy grail!
My personal preference would be to go for the 16mm master for better feel and more power. A 19mm is probably going to feel woody and not exert so much pressure.? (I have no experience of later brakes with larger masters, I can only assume that they are intended for bigger caliper pistons or more pots.)
Another factor in the mechanical advantage of a hydraulic system is the lever itself. The closer the pivot pin to the piston actuator, the more leverage it will give for more lever travel. I believe this leverage moment is adjustable on Radial masters? So you can get back the leverage that has been lost with a larger master piston.
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