Always, always pre-wash flannel. Even if you don't normally pre-wash, they shrink and at different rates.
I would suggest no tiny pieces or not a lot of triangles that the seams all meet, it would be to thick with a lot of seams coming together.
Flannels make great rag quilts, I have also done a couple in Yellow Brick Road, and have used several other patterns that don't have tiny pieces.
They're not hard!
Have fun!