Edgers have typically two (2) standard sizes:
 
Blades for gas equipment are 9-1/2"
Blades for electric equipment are 7-1/2"
 
The edger blade has been constructed to fit all gas-powered equipment, without any modification.
 
For ELECTRIC edger equipment, here's how you size it:
 
Measure your existing steel blade, from the center hole to the tip of the blade. Then, measure the same length from SharperBlade's center hole, marking the measurement on the SharperBlade (i.e. 3-1/2"). 

With your garden sheers, cut along the measurement line you just drew.

Choose the appropriate "centering" washer for your Edger and mount the SharperBlade using your existing bolt and washer.