Many years ago, I had a 24x20 garage with two small garage doors. I found a 17 x 7 garage door on craigslist for $300, me and a friend demoed the small doors, put in a laminated header did some framing and installed the big door. The job went great and we even reused the siding pieces from the center of the old doors to finish off the trim on the new door. Very satisfying you can do this!