How big do Staffordshire bull terriers' necks grow?

This really depends on the weight and height of the individual Dog. Some bloodlines are leaner than others and so will have thinner necks, whilst some bloodlines are broad and stocky. Most Staffords will have a neck in the region of 18 and 22 inches wide. My 3 year old female Stafford has a 20 inch neck, but every Dog is different.