The width of the Jelly Roll Race Quilt is completely determined by the length of your initial mega strip. Assuming you would like a horizontally striped quilt, we can easily change the width by adding or subtracting the number of strips we add to our first mega strip. It is the length that gets a bit tricky. Usually the fabric strips in a jelly roll are around 42 inches wide. We will assume some loss due to the selvedge edge and account for a 1/4â³ seam allowance on each side, so we will round each strip width to approximately 40â³.