For my square I have used a size 5mm hook with merino soft. Because it is double crochet, it's a much smaller stitch with less stretch. You need to pay extra attention to your gauge for this square! Oh and it'll curl! Even once blocked you may still get a little curling of corners. Once joined to the other squares it'll be fine. Please don't miss out the blocking process. It really will make a difference.
The chart provided with the pattern tells you where to decrease on those dreaded border stitches ;-)
Take a little time to write it down....it'll save time in the long run.
You can see my square in my Dancing Around the Beach Huts colour scheme along with the grey merino I'm also making a blanket in. Making both blankets is keeping me very busy!!!