lets start by throwing out the size or material of the ferrule of the broadhead and focus on the blades of the broadhead which do the cutting. i am gonna use a (2in. cut with 2 blades), (1.5in cut with 3 blades) and a (1.125in cut with 4 blades). the 1.125 in is the same as 1-1/8in cut. the size of the blades would always be 1/2 the diameter. here goes

2 inch 2 blade would be 1 inch for each blade = 2 inches of blade.
1.5 inch 3 blade would be .75 inches for each blade = 2.25 inches of blade
1.125 inch 4 blade would be .5625 inches for each blade = 2.25 inches of blade

don't be fooled by the diameter of the cut.