If you have 2 mirrors handy that you can stand side by side on the fabric you can see what it will make. Stand them so they make a triangle shape on the fabric...about the size you are planning to make your cuts. This helps a lot with deciding the fabric.
Also the repeat needs to be at least 12"....and if I recall you need about 5 yards of fabric to make a twin size...not sure about that...only if my memory is working!