This badge is 7 cm in diameter (it fits just inside the palm of your hand) and can be sewed on to your backpack or jacket.
Why a forum badge?
You do not need a badge to be a pilgrim.
The idea behind this badge is for you to get in touch with other fellow forum members while on your camino.
Also, "everyone" in Spain will know that you are a pilgrim when they see this badge. Your backpack might also tip them off
More on the badge from the forum here.