Below is the dimensions for a quarterback target. The target is an 18 inch square with a shirt inserted. I prefer a quarterback throw at a shirt target because it simulates throwing to a person and aiming at a shirt the exact size of a person.  If you throw at a net it is not the same object for the quarterback's eyes to look at as a receiver.  I believe it is important to train the eyes to look at a specific spot on a receiver and to throw to that spot. And, if he misses then he misses the whole target. Using a net, if the quarterback misses the target on the net, he still hits the net. I believe this gives the quarterback false positive feedback of having hit something. I want him to throw at and hit an exact replica of a receiver's body and know that when he misses it was not a good technique throw.