Just to get you started, it is not all that complicated. As a general guide, the force required will be (fairly) constant for the portion of metal in shear. How wide the material is does not really matter to any large degree. What matters is the length of material being torn apart (cut/sheared) at any given time and the maxium thickness at the start.
Shearing is not cutting per se, think of it as pushing one piece of metal past another, you just have to be breaking the moving piece off the fixed piece and that's where the force comes in.
Do you have a copy of Machinery's Handbook. All will be revealed.