That link doesn't show the pattern, just the page to buy the pattern. Did you already purchase it? If so, you can easily change the numbers even if you've never done it before... you just have to make proportional changes for the bigger size. For instance, if the L says to CO 68 stitches, and the XL says to CO 76 stitches, the next size up would be to CO 84 (there's 8 stitches b/t each size)... if you needed to go up one more it would be 92.
You can do the same with figuring out yardage. If you want to PM me your measurements, and the pattern, I can totally help you. I'm used to biggerizing patterns! I can walk you through the process, and check your math.... I'm all about knitting math, too!