Its written on the side of it. IIRC it's 1/2" ( or 12.7mm )

I did the calculations for caliper/master cylinder piston ratios some time back.

STANDARD master cylinder to STANDARD 2 piston caliper ratio is 15.375:1

STANDARD master cylinder to 3 piston caliper ratio is 23.374:1

CBR600F master cylinder to 3 piston caliper ratio is 14.949:1 ( almost the same as standard )
The CBR600F master cylinder is a 5/8" piston ( or 15.88mm ) and it matches the CBR1000F master cylinder for looks.

CB1300 master cylinder to 3 piston caliper ratio is 19.234:1
The CB1300 master cylinder is a 14mm piston and it matches the CBR1000F master cylinder for looks.



EDIT --- Just been looking at some other master cylinder to caliper ratios :

CBR1000F ( Pre CBS braking system - 2 pot calipers ) 16.00:1
CBR1000F ( CBS linked brakes ) 15.375:1
CBR1000F ( STD master cylinder - 3 pistons linked in caliper ) 23.374:1
CBR1000F ( CBR600F master cylinder - 3 pistons linked in caliper ) 14.949:1
CBR1000F ( CB1300 master cylinder - 3 pistons linked in caliper ) 19.234:1
CBR1100xx ( CBS linked brakes ) 15.701:1
CBR600F ( 4 pot calipers ) 12.69:1
CBR929RR ( 4 pot calipers ) 22.918:1

I was hoping to find a common ratio for linked and a common ratio for non-linked but that is not the case.

Personally I would be fitting the CB1300 master cylinder and linking all 3 pistons in the front calipers