I want to put dummy strap hinges on our plain 9 foot wide overhead garage doors. How long should the hinges be, is there a formula?
Use your own best judgement but not until you do a little work. Sketch the side of your garage with the garage doors to scale. Copy this drawing several times and overlay hinges of various sizes on them. You should be able to tell which sizes just don't look right, either too large or too small.
Pick the one that looks right and determine the scaled size of the hinge. You should be able to find something in the ballpark of that size.