How can I compute the beam diameter of an image?

I have a set of ccd images of a laser beam and I want to be able to compute the beam diameter (in mm) from each image. Currently, I'm thinking of something along these lines:

pixelsizemm = 4.65*10^(-3);
image = imread(FileName,'bmp');
imsizepix = size(image);
imsizemm = pixelsizemm*[size(image,1),size(image,2)];
BI = image>75;
newBI = bwconvhull(BI);
imshow(newBI)

to take the image and transform it into something close to a circle, but I'm still unsure how I can go from this to calculating the diameter of the beam, or whether this is the best way of doing it. The beam profile is also doughnut shaped, meaning that the binary image is sometimes picking up that there is a black hole in the middle, which I wan to ignore. I've included an example of one of the images I need to analyse.