You can do them any size you want. Here's the math:
9 squares across + 2 sashing's between the 3 columns + borders.
Use charm squares (finish @ 4.5") and 1" sashing & inner border, your quilt is 44.5" plus the outer border(s).
Using 3 1/2" finished squares & 1" sashing/inner border, it is only 35.5" before the outer border(s).
Using 6" finished squares and same sashing/inner border scenario, it is 58" before any other border.