The 4-pot 65mm Brembo caliper has 2 pistons 34mm diameter and 2 pistons 30mm diameter. For purposes of using 3M's chart on the 650Central website to identify an appropriate m/c bore diameter, this is equivalent in face area to 4 pistons 32mm diameter. The chart shows a 1/2" bore is very close to the "sweet spot" for a single-disc system using the 65mm Brembo caliper. I use a 1/2" bore Yamaha P/N 2UJ-25870-00-00 as supplied on 1988-90 Route 66, 1995-2007 XV250, and 2008-2011 VStar 250 models; it's virtually "plug-and-play" on later XS650's.
Well, I have a 95 Ducati Monster with a single disc and the "40 mm" Brembo 4 piston caliper, which has pretty much the same total piston area as the "65 mm" calipers. And the stock Brembo master cylinder for that bike is 13 mm. The brake on my Monster is very good, so you should be perfectly fine with either a 13 mm or a 1/2" master cylinder for a single Brembo 4 piston caliper, either it is a "65 mm" or a "40 mm"
