First of all, I think the pattern you have chosen to do is a great choice for the flannels you are working with. If you want the best finished product possible there are steps to take. Some flannel shrinks a great deal. It depends on the quality. I have had some flannel (name brands like Moda and Maywood) that had little shrinkage. Others, from chain stores were loosely woven with yarns that were not tightly twisted, shrank a lot. Because you have a variety of pieces it is important to get the shrinkage out. Hot water and an iron will take care of the problem. Use the hottest water out of the tap. Fill the sink. Add the fabric pieces (you might want to use a lingerie bag for some of the smallest pieces). Let it soak for a while. Drain the water out, squeeze (don't wring) the excess water out. Dry by hanging on hangers or a line and iron with a hot iron. I like to use steam when I iron them. Be sure you wait until the pieces are completely dry before storing them. Just be a bit patient and you will be fine.