When you join a jelly roll together you have 1600 inches of fabric hence the name 1600 quilt! So with your 1600 by 2 1/2 inches you fold it in half to create 800 by 5 which you then sew. Then you repeat the process until you are left with 50 by 80. That is how I made my 1600 jelly roll quilt.
I have just been asked to make a quilt from a jelly roll. The customer had purchased fabrics from the Ocean Wave and Summer Days range by Fabric Freedom since they are so vibrant but didn't know what to make.