My advice if you do use a sheet as a backing - this is from personal experience - buy a sheet that is much bigger than your quilt top! The first quilt I ever made was for a friend's daughter. It was for a double/full size bed. The friend wanted a flannel back/sheet and was planning on finding it and buying it herself. I told her to buy a QUEEN size sheet because I knew the flannel would shrink A LOT! Well, she didn't. She brought me a lousy quality flannel in a double size. As I'd prewashed the top fabrics already, I knew I had to prewash the flannel sheet and I knew it would shrink (and warp!), but better now than after it was sewn together. Did it ever shrink (and warp!). I ended up having to cut a lot off the quilt top to accommodate the sheet backing. I was stressed and mad. Even a cotton sheet will shrink. Better to have too much fabric to work with than to end up short and then stress about how to salvage your efforts!