The tracking process is able to detect "blobs" in an image which represent multiple particles in a number of ways. This option sets the the upper size limit for a blob. If the area of a blob exceeds this limit, then DigImage can be told to either ignore the blob, or consider the blob to represent two particles (see the particle tracking overview for more details).
Simillarly, multiple particles within the same blob may manifest themselves by the ellipticity of the blob exceeding certain bounds. [E Ellipticity limit] sets the limit on the ellipticity. Again, if this limit is exceeded, DigImage may either ignore the blob or consider the possibility that the blob represents two particles.