That IS a super cute blanket! I saved it to my iPad!
The reason they suggest that needle size is because the yarn used in the pattern is a sport weight. If you use a larger needle on that yarn you'll get a loose or possibly lacy fabric depending on your own style. So if you use a heavier weight yarn a larger needle would be fine. Keep in mind the blanket will be larger that way, but that's probably no big deal for a blanket.