Hello Willem,
just make a Particle Analysis in ImageJ to measure the area of all "spots":
http://rsbweb.nih.gov/ij/docs/guide/146-30.htmland then summarize the results in the Results Table to get the size of the largest particle (Max):
http://rsbweb.nih.gov/ij/docs/guide/146-30.html#sub:Summarize