You can use minboundrect() in John D'Errico's function http://www.mathworks.com/matlabcentral/fileexchange/34767-a-suite-of-minimal-bounding-objects
Or else if you don't want the bounding box width, and want the actual width perpendicular to the major axis at the mid point, you can use my attached demo. regionprops() does not give the actual feret diameters, just the axis lengths of an ellipse fitted to the blob.
