I'd consider 12' about the minimum width. More is nicer, of course; but that's true pretty much no matter how wide you make it. For the height, it's extra convenient to be able to do RV roof maintenance while under the shelter, but that may not be especially practical or look at all in appropriate scale with a ranch house.
Keep in mind in your planning that you'll have to drive into and out of the shelter. If you can't get aligned perfectly with it before entering and exiting, you'll need some extra space for maneuvering and turning/angling as you go. Twelve feet wide calls for a good bit of precision in parking when it's delineated by support columns rather than painted lines; and all the more so if the space between the supports is actually a few inches less (and the overall external width is 12'). Along those lines, if the approach isn't level, you need some extra height to account for the tilting of the RV as you enter and exit.
If you think you might ever upgrade to a larger RV, planning ahead on the shelter size is worth considering. The maximum height for a vehicle in the US without an overheight trip permit is 13' 6", so if at all possible I would suggest a shelter that can accommodate that height. Many class A's and fifth wheels are that height, or very nearly so.