I love using drop cloths for home projects. They're a great way to get lots of heavy cotton fabric cheap, but there are a few things to keep in mind:
1. The size they list on the package is approximate. Mine was probably 6' by 9' before the seams and hems were sewn, but were a few inches shorter when I measured them.
2. Don't think you're getting a single large sheet of fabric. My cloth had a seam running the 9' length, but they can put a seam
anywhere they want - it's just a drop cloth, after all! If I want yardage that I'm going to cut and re-shape, I buy the 4' by 15' size. Every package I've bought of that has been a single sheet of 48" wide fabric. After washing, it's been about 47" wide by 13 feet long.
3. Beware of ironing the cloth before you wash it. There could be some really gunky sizing on the fabric that comes off on your iron. This is the voice of experience speaking here. Luckily, it washed
right off the iron after it cooled, but still....
4. Of course, the hems around the edges won't necessarily be straight. You may have to cut and re-hem. I did on the top and bottom of my curtain, but the sides were straight enough for me as is.