When I do backstitch on a flower like the one in your design, I try to keep the backstitch consistent. I usually don't go more than 2 or 3 aida squares to do a single backstitch. So if the flower side is 4 stitches long, I would split it into 2 backstitches of 2 aida squares each. It looks like the long sides of the other petals are shorter in length, but if there are any that are as long as that longer one you are backstitching, I would do the same length by splitting the backstitch to cover 2 squares each for the 4 stitch length. There are no cross stitch police, so experiment and decide what you think looks best and keep it the same for the other petals of the flower that are the same length.