I bought two yards for a 120" 2.35:1 screen and it is stretched very tight and barely made it. I have silver over white and I bought three yards of the white locally and it was much easier to attach. I would agree that three yards is ideal.
I used 1"x3" poplar boards from Lowes that are available in 10' lengths. My 120" diagonal screen at a 2.35:1 aspect ratio is about 111" x 47". Make sure the spandex you order is 4-way stretch. Also, the higher the spandex content the more it can stretch.
As noted on other spandex threads 1x2's wont cut it. Your best option is getting 1x(3 or 4)x 10 for the top and bottom. Actually four 1x4x10 should get you top, bottom, 2 sides and 2 vertical supports on the middle. I highly recommend checking out all the spandex threads, you will learn alot.