What size collar does a 3 year old Staffordshire bull terrier need?

Staffordshire bull terriers are a medium-sized dog breed and a 3-year-old will typically weigh between 11 to 17 kilograms (24 to 37 pounds) and have a neck size of between 30 and 38 centimetres (12 to 15 inches).

Therefore, you would need a collar with a width of at least 2.5 centimetres (1 inch) and a length that is adjustable between 30 and 38 centimetres (12 to 15 inches).

It's important to note that these measurements are just a guideline, it's always best to measure your dog's neck to ensure the best fit.