Bonnie Hunter has turned this idea into a science. I happen to have her book "Scraps & Shirttails: Reuse, Re-purpose, Recycle! The art of Quilting Green" out of our guild library at the moment, and it inspired me to go out and buy a pile of the biggest men's all cotton shirts I could find. I have just started cutting them apart, and have been impressed with the amount of fabric that came out of them. Two were size XXL, and they were 28 inches across the back. I would suggest having a look at this book if possible because it is full of useful suggestions and some very attractive patterns.
A few years ago I made a rag quilt out of recycled flannel shirts and denim jeans. My daughter was thrilled with it because it's very heavy and warm as well as being an earth-friendly alternative to buying all new material.