It depends on how big you want to go. You can use thicker yarn and larger needles with the same pattern for a larger size, or you can increase all the stitches in the directions by a certain percentage.
If the pattern comes in various sizes, you can check to see how many stitches are increased for each size and increase the same amount for your size.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry