Use any scale you like for sashing/cornerstone dimensions. In this illustration, each finishes with a width of 3". That means you would cut 3-1/2" x 15-1/2 sashing bars and 3-1/2" x 3-1/2" cornerstone squares.
Yardage requirements will vary, depending on the number of blocks you make. Use my How to Calculate Yardage instructions to determine fabric requirements.