Someone had a question about lap quilt size and another person posted a response showing the sizes of bed quilts. If you look at that and add a few inches (to be safe) all the way around. You should do okay. Most fabric is 44 or 45 inches. You will probably have a seam, but most backs do unless you go with really wide fabric such as 60 or 90 inches. This may help you a little, but I am sure someone more experienced than me can give you an even better answer.