Clouds
Angular measurements in some cases could roughly estimate the size of a UFO.
If there are clouds or cloud cover during the sighting, you should be able to determine their altitude that day. Obviously the object would be below the clouds for it to be seen so using the angle above the horizon and the distance from the ground to the cloud cover, with a little trig, you could calculate the distance to the object.
With something like your "closed fist" example to get the objects angular size, you should be able to come up with a range of possible sizes.
Just a thought.