I think the ideal size would be 9 x 12, which would look something like this:
(That's a step in the lower right-hand corner, coming from the house.)
This Restoration Hardware rug (in brown) is my favorite out there, but the largest is 8 x 13, which isn't the right size for my space. It would be a few inches TOO LONG, so we'd have to turn it the other way.
The next size down at Restoration Hardware is TOO SMALL- 7'6 x 10. I also like this one from Ballard Designs, which is slightly bigger than that at 7'10"x10'10", but it looks STILL TOO SMALL on our porch:
Home Decorators is also a go-to source for me because they often have free shipping and good coupons, but their sizing doesn't work either- mostly 7'6 x 10'9 or 8'6 x 13. This one is 7'11 x 11 (slightly bigger than the Ballard Designs rug, but STILL TOO SMALL).
Crate and Barrell does have a few 9 x 12 rugs, but they're a little boring, and I'm not sure how they'd look.
This one's on sale.