Twenty-one different squares may be selected.
Of these nine will be of the size shown by the four A's in the diagram, four of the size shown by the B's, four of the size shown by the C's, two of the size shown by the D's, and two of the size indicated by the upper single A, the upper single E, the lower single C, and the EB. 
It is an interesting fact that you cannot form any one of these twenty-one squares without using at least one of the six circles marked E.