If the pattern uses the fixation yarn which is dk with size 5 needles, use a worsted and size 7 or 8 needles. You probably only need to make it a couple of inches larger. Here's a couple charts to head sizes - http://www.craftyarncouncil.com/headsize.html http://bevscountrycottage.com/size-chart.html#hat
- and here's a pattern for hats
using any yarn or needle to fit any head size.