Target size represents a percentage of the area of the VMD window that will cause the camera to trigger a clip. If the target size is set too large, it will only pick up very large objects such as a vehicle. If the target size is set too small, objects such as leaves blowing or small animals will trigger a clip.
You need to adjust the target size based on the location of the VMD. A VMD close to the camera itself will usually require a larger target size compared to a VMD that is at the far from the camera. For example, when you stand at the far end of your driveway, you’ll look smaller than when you’re standing in close proximity to the camera itself.