There seem to be 3 stitches between each size. You could try to increase every number by three.
Or you can try going up a needle size and see if the fabric is acceptable to you--that will enlarge the pattern. I can't say how much, exactly, but I did that with a child's sweater because it was recommended in the pattern, and it did make it bigger by a size, at least.
"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