It was 1.5" square (not that I've done anything with those yet)... then I found a great Anita Solomon Grossman (or vice versa on her last name... can never remember :) ) pattern using 1.25" squares - so that was the minimum. Then I found the crumb demos....
My goal is to actually make a crumb quilt. So now anything below .5" is too small. Anything above that can go in the crumb quilt and be seen after seam allowances.
The best reason I started keeping my scraps was when someone pointed out scraps are still $7-10/yard too.