As far as the size goes, just grab some spare bolts from your stash and see if they thread in by hand. To get the length measure the thickness of material you are bolting, that is the grip length. take a piece of wire and measure the hole depth, get bolts that are short enough to not bottom out in the hole before your bracket is snug. If the bracket is 1/8th " thick and the hole is 3/4 " deep we then know that a 1 inch bolt is too long by an 1/8th" of an inch and should be a 3/4 length bolt so there is some wiggle room. When you figure this out you can go to napa and get the bolts with the flange washer on them.