How I do mine is a cut me three strips the size I want. After sewing the strip set together I measure the width of the set and cut my squares that exact size. Works everytime. I like to use 3, 3 and 4.5 strips and 9.5 squares. If you sew with an accurate 1/4 seam you just subtract 1" from your cut strips to get the size of your squares. Another size I have done due to a beautiful large print is 4", 4" and 5 1/2" with the square being cut at 12 1/2". You do have the correct sizes for the pattern.
I hope this will help. Have fun it is an easy pattern.
Connie