Video motion detection software

Hi, I'm looking for a video motion detection software where I am able to define an target size and get an alert when objects is bigger than the target.

I use a Axis Q1635 which has this function, but the problem is that it's not able to "see" every object as different objects, most of the objects shows as one larger object.

The camera is installed at the primary crusher in a quarry, and my costumer want to get an alert when to big stones enter the crusher.

I recently tested iSpy, but by the looks of it it lacks the function to define a target and detecting the rocks properly.

The picture shows what the camera sees.

Does anyone have any suggestions?

Have searched a lot, but was not able to find anything else that looked promising.

Thanks a lot for any answer!