1 fat quarter = (approx.) 18" x 22"
18" can be divided by 2", 3", 4.5", 6", and 9" evenly.
21" can be divided by 2", 3, 6", 7" evenly.
I'd suggest cutting your squares at 6" square which will finish approx. 5.5" square. You'll get 9 squares @ 5.5". square from each fat quarter.
Set your quilt at:
11 squares across will = 60.5" wide
15 squares down will = 82.5" long. You will need 165 squares, from 19 fat quarters.
To enlarge that, you could add borders.
Jan in VA