OMG, that sure is a cute pattern!
You'll want to make a guage swatch first. The pattern says 16 stitches = 4". So cast on about 25 stitches and knit for 5 inches then measure across the center 4" and if you get 16 stitches, you are good to go. Switch needle size up or down one size if you're a bit off.
It also says to swatch in the round. Here is some info how to do that: http://knitting.about.com/od/knittin...lar_swatch.htm