i think the 'traditional method' on this site tells you what your measurements should be. great question! i've always wondered myself!
"To make a Flying Geese unit following the Traditional Piecing method, you need one SQUARE, the size of the finished width you desire it to be + 1 1/4", AND two TRIANGLES cut from squares that are the height of the finished unit you want plus 7/8". Cut the large square on the diagonal twice. This will give you four geese. Then sew a triangle to each of the top angled sides, using scant 1/4" seams. You will get four Flying geese units per large square.
EXAMPLE: to get four 2" x 4" finished flying geese units, you would use a 5 1/4" geese square cut twice diagonally, and four 2 7/8" sky triangles, cut diagonally once each."
http://www.quilterscache.com/H/Howtomakegeese.html