Mustang Master Cylinder sizes

Model			Bore		Primary L-R	Rear
79-86 V8 Mustang   	21mm (7/8") FF	9/16-18 SAE	1/2-20 SAE
84-86 SVO/Lincoln      	28mm (1 1/8")	9/16-18 SAE	7/16-24 SAE
87-93 V8 Mustang   	21mm (7/8") FF	12mm-1 ISO	10mm-1 ISO
93 Cobra           	25.5mm (1")	12mm-1 ISO	10mm-1 ISO
93 C-R (F3ZZ-2140-A?) 	25.4mm (1")	12mm-1 ISO	10mm-1 ISO
94+ GT (& GTS)     	27mm (1 1/16")	12mm-1 ISO	10mm-1 ISO (hole in webbing)
94+ Cobra          	24.5mm (15/16")	10mm-1 ISO	12mm-1 ISO

Smaller equals softer.  The smaller the bore, the farther the piston needs
to move to displace the same amount of fluid as a larger bore.  Like a
lever, the farther you move your end, the less force it takes to move it,
assuming everything on the other end of the fulcrum stays the same.