First I cropped a series of images and then resized them to be approximately 8 inches tall. Of course this makes the flower many times larger than in real life, but they need to be bigger to be piecable. I got to the decision on 8" by doing a few trials, seeing what size some of the individual pieces would need to be and if that looked achievable without a lot of grief. And I also played around with some rough layouts to see if that size would let me showcase a variety of flowers without making an enormous quilt.
Note that this flower is very different from the others, pale pink, lots of veins, drooping and with no dark pink sepals.