This is on the website of the link posted above:
Making Geese in Any Size, The Formula:
You can make your Flying Geese in any size using the following formula:
Size of squares = 1/2 inch larger than the desired finished height of the overall unit.
Height of the rectangle = same as the squares you cut.
Width of the rectangle = width of square times two minus 1/2 inch.
In other words, if you cut 2" squares, you would cut the rectangle 2" x 3.5"