M-fay - I just randomly sew my small scraps together to make fabric big enough to use. I usually end up with a 4.5 or 6.5 inch square. The red scraps are what I'm working on now and the picture of my design wall is a UFO project I'm doing.
Doing crumb block is just mindless random sewing, very relaxing. But you get exercise if you keep your iron in a different room, since I press after each addition.