You can get the Equivalent Circular Diameter from regionprops(). This is the diameter of the blob if you were to smoosh and mold that same shape into a perfectly circular shape. In other words, the diameter of a perfect circle with the same number of pixels as your irregularly shaped blob.
props = regionprops(mask, 'EquivDiameter');
% Extract it from the structure into a simple variable.
ecd = props.EquivDiameter % This is the "Equivalent Circular Diameter".