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I always make the RO the same size as the door. That way when finishing with 1x material it covers up the unfinished metal edge off door. Then I add seal material around door to keep the elements outside where they belong.
Generally, I will make the rough opening for a 9x7 garage opening 9'3"x 7'1 1/2". The house I frame then get padded in with 2x8 clear cedar. A 2x8 would be for a 2x6 wall therefore you would have astop for the siding. It would also be the same for a 2x4 wall having a 2x6 jamb. The cedar might cost a little more but it holds up well against the elements. Then with the weather stripping the edge between the door and the jamb is sealed. Hope this helps.
Stan here is how I frame a garage opening on the west coast, for a standard sectional roll up door. No arches, standard jambs.
What you want to end up with is a finished opening just under 9'. This will cover the edges of the door. Finish jamb to finish jamb you should have 8' 11 1/2. To get this use two trimmers on each side of the header plus the finished jamb. I use 2x 6 or 8 rough spruce/hemfir on a standard opening. The two finish jambs together make 3 ". Use two trimmers on each side, together they are 3 1/16 ea side making a total of 6 1/8. Add the 3" for the jambs for a grand total of 9 1/8". I cut a 9' 8 5/8 inch 4 x 12 garage header. To get the proper head height, measure from the finish garage slab floor to the bottom of the header (without the finish jamb)7' 1". It could be 7' 1 1/2 but I drop it down the 1/2" to eliminate the little 1x finish strip. Remember not to measure from the stem wall or the header will be way high. Once the R.O. is filled in with the finish jamb the heigth is 6'11 1/2. How you position the finish jamb is determined by the siding and how it is being trimmed out.
Stan its not what the garage door people will allow, but the codes and that depends on the load above. Tile roof , snow loads, high winds create the factors to determine the carry capacity needed. If you have normal load, asphalt roof, a 4 x10 should work fine up to ten feet.
Dunk, are you sure about that? For all we know Stanley is framing the opening in a one-story gable end and has no header at all. Seems like the garage door manufacturers will want some minimum header to back screws for track supports, torsion spring pad, etc. Or maybe I'm wrong---maybe the 2 x 6 "H" they want on the inside of the opening is enough backing for everything.