Geeze, with WoolEase chunky (or thick and quick) you should be able to go up to size 10.5 or even 11 needles and follow one of the pattern sizes they have.
There's a couple ways to fudge the pattern.
Use another size, like I suggested above. You figure out how many stitches/inch you get with your yarn and needles, and multply that by how many inches around you need. See if that number is close to the total stitches for front and back for one of the sizes listed in the pattern. You may have to adjust length on body, sleeves and armhole, but most patterns tell you knit for X inches anyway, so just make sure you use your tape measure frequently.
The other way is to look at the number of stitches difference between sizes. Say for the Large you CO 70, and for 1x it's 78. So if you need a 3x, figure that's 2 sizes larger than 1x and you need 16 stitches more than that. Check to make sure that will be big enough as in the first example above. If that's how you want to go, do the same for the sleeves and everywhere it mentions number of stitches.
ETA: I just realized that it's a free pattern and actually looked at it. D'oh. You could use size 11s I bet, and use a combination of larger needles and the stitch/size ratio, too.