How much larger than the largest size do you want to increase it? Just a bit or a lot? If just a bit you can usually ascertain the ratio between the existing sizes (example: CO 80 sts for sm, 90 for med, 100 for lg....thus the diff is 10 and an xlg would need CO of 110). That's just a rough gauge.
If you know your gauge and the number of inches you want the final item, you can input the respective info into this quick and easy program:
and it'll compute new numbers for you. Make sure to utilize the 'Shaping' option for necklines, incs/decs on fronts/backs, armholes, etc.