You crumb quilt is lovely! I'm currently working on a crumb quilt myself, using crumb blocks that are 6-1/2 square and hope it turns out half as nice as yours.
I think a crumb is anything smaller than you would find useful, so it can vary from person to person. For someone who likes to make big block quilts of applique, anything smaller than 12' might be a crumb. In my case, I like to do scrap quilts, so anything that cannot be cut into at least a 2"square or a 1x3" strip is a crumb. Anything under 1" gets chopped into confetti to use as filler for pet beds.