I recently scrounged some steel shelving that I'd like to hang in my garage. This used to be in an office, its for paperwork but will work great for some things in my garage.

The mounting holes on these shelves are not going to be centered on my studs. What I think I need to do to hang these is to screw two boards to the studs and then mount the shelves to that.

How do I determine what size of board to use? Is it based on size/weight of the shelves? 

The shelves are basic steel box shelves. They are roughly 4' long, 10" deep, and 12" tall. It's a steel box open to the front and there is a slight flange on the back.

The garage wall is finished. Just has drywall and paint.