I have made quite a few bargello quilts, I enlarge mine when needed by using graph paper and drawing the next row out, then the next and so forth, It does take time but I know where I am going this way. I can see the center is cut smaller so stay with the given measurements all the way, then at the end of the pattern you may want to go a little wider as the pattern you have now states. The only problem I would be concerned about is the next color on the width because it is a jelly roll. If I am not mistaken, (which I could be because I haven't studied the pattern ) The length should just be adding more and the stepping out of the colors. I may not have helped but it has worked for me different times. Good luck.