Always keep in mind that you are buying a bean bag door stop to prevent the door from banging the wall. You also need to be aware that bean bag door stops typically compress when pressure is applied. This means you can't just buy a bean bag door stop that has the same thickness when compared to the thickness of the doorknob. This is because the bean bag will compress, and the doorknob will still likely hit the wall. Hence, you need to give it a suitable buffer.
The other thing that you need to consider is weight. Generally, the heavier the bean bag door stop filling, the lesser it will compress. In other words, you can get away with a smaller-sized bean bag doorstop if it has a heavier filling. On the downside, heavier fillings tend to be more expensive than the lighter ones.