It looks as though the hat is done in stockinette stitch with a ribbed band. If that's the case, you only need to know your gauge and cast on the right number of stitches for the size you wish to make. For the ribbed band you'll need a multiple to come out right for the ribbing. Is it k2, p1? I can't tell for sure. For k2, p1 you need a multiple of 3, for k2, p2 rib you need a multiple of 4. Others make lots more hats than I do and will be more able to tell you about how to space your decreases or how many to increase if you need them too. You might take a look at www.ravelry.com
to look at similar hats done in the weight of yarn you want to use. Many of the patterns there are free. HTH and happy knitting!