For me, a crumb is anything small enough that a sane person would throw away instead of saving. But then, I'm a quilter so by default I'm not exactly the sanest person around LOL.
I keep a basket by my cutting table just for all those odd shapes and small pieces (anything over 1.5") and periodically as the basket fills up, and I'm in-between projects or don't know what I want to work on next, I just start randomly reaching in the basket and sewing pieces together. When I get a big enough collage of fabric sewn together then I cut it down to a 9" block. When I have enough blocks they get sewn together. I find the process very relaxing because there's no thought process invoilved: no measuring, no planning, no pattern... just sew and trim.