The length of the snuggle blanket will be equal to the length of the back measurement you took. To figure out the width of the fleece snuggle blanket, simply add about 6-inches to the chest measurement. Cut your piece of fleece using these numbers.
For example, my chihuahua's back was 15-inches long and his chest measured 17-inches around. I would then cut a piece of fleece that was 15-inches x 23-inches. I initially only added about 3-inches to the chest measurement and the snuggle blanket was a little too snug so by adding a few more inches you should be good.