Are you measuring exactly??? Now....after checking that -- start a new one...(smaller will help you learn).
first -- where did you get the pattern? Not to put any site down, but I've found problems with some on the websites for patterns.
When you sew -- do NOT stretch your fabric. If you remove stitches, do NOT stretch your fabric. Sew two "blades" of the star together, then two, then two, etc. Now sew 2 of the doubles together. Then sew 2 more of the doubles together. You should have 2 pieces...4 blades each. Iron the seams in one direction. It helps me to lay the pieces on the ironing board as if they're a whole star, right side down. Iron CAREFULLY - do NOT stretch! Now sew the two pieces together.
I cannot stress enough to NOT stretch!
Good luck!