That is going to be a beautiful quilt! Very cheery.
Looking at the pictures, the two on the left look like the same size and the one on the right bigger. Is the center square on the right one the exact size as the other center square. Could be the angle of the picture but it looks a bit different which would throw off the size of the square. Also, if you do all of your squares at the same time attaching the identical size piece to the identical size of the previous strip and strip piecing as the others have said you will notice right away if you make a little oops!
Have fun with it and make sure to post a picture when you have it complete.