I like a quilt I saw at a winery. It had the legs cut off the jeans, the seam removed and flattened out. A second leg from a pair of jeans was oriented in the other direction and so on. It looked like stacked trapezoids. If you have enough pairs of jeans to recycle, the quilt can be the inseam length as the width and about 50-60" long, pieced like this on both sides. Or you can use some other heavy fabric on the reverse. Batting would be optional. The width could be doubled by using 2 legs, and the length made as long as you want.
I made a queen size sampler quilt from denim. I wouldn't recommend that. The denim is hard to work with. I used different colors of denim, so the fact that some of the denim was new didn't help. I used 9" blocks. I had a rail fence block, a pinwheel block, a sailboat block, a 9 patch. It was even harder to patch. It survived a hurricane that destroyed our home, with some damage to the quilt.