You can use any size square you want - it is your quilt, your choice! Especially if it is a bigger pattern, you may not want to cut it up in small squares. A suggestion for fat quarters - being 18 x 22, how about 9 x 11. It would be easy on the eyes because it is a size we are used to (being about the size of notebook paper) and would give you 10 pieces. This is a nice number to either use as is or to sash. Please be sure to post whatever you decide to make.