Woody,
Have you come across the Whims website?
http://www.whims.cc/
They sell pre cut squares, patterns,have some video tutolrials and everything else you need to make a watercolor quilt. They sell the small squares and those are the ones that I imagine work best for this kind of quilts. They also sell fusible grids where you fuse all the squares the way you want them and then just stitch on the lines. I have never made a watercolor quilt personally but have been collecting squares for one.
Good luck.