Heather137,
LucyInTheSky is right. Backing size depends on your top size.
My daughter, the artist, has recommended a border that is a multiple of the block size. So I might do a 3 inch border (1/3 the width of the block). Assuming that measurement, you would cut it 3 1/2 inches wide to start. When you add it to your top, you will have a top 69 1/2 " square. (border yardage - 7 strips, 3 1/2" wide = 3/4 yard)
As to the backing:
With any quilt - determine if it is larger than usuable width of your fabric. If you have 40" usable, than divide your width by 40 to determine the number of lengths of fabric. (In your case, it would mean 2 lengths for a 69 1/2" width.) Purchase enough fabric for extra length. I would recemmend 6" (3" per side). For this project, that would be 75 1/2", two times, or 151" (4 1/4 yards, rounding up).
That being said, there are backings available in widths of 108", and you would only have to get 1 length plus the 6" or so extra. But that would leave you with a lot of extra fabric.
Happy quilting. I know your quilt will be great :!: