14 mm piston bore is the right size for a single disk, Dual disks will have a larger bore to move more fluid ,going to a smaller bore on a single disk will increase the firm ness on your lever up to a point,12 mm seems to be the lower limit for a front brake ,some bikes there rear master cylinder will be as low as 10 or 11 mm bore size,your 14 mm is just about right for most applications for a single disk, it should be firm without excessive lever travel,by going to a larger than 14 mm bore size you will increase lever travel and your lever will have a softer feel as you will be having to move more fluid,16 mm is a dual disk size
