In general you can figure the aproximate adult weight of a dog by doubling the weight at 4 months. With a 12 week old puppy you can double it and add maybe anywhere from 2-10 pounds depending on the weight you get on the 12 week old. Low weight you add less than on a higher weight.
Here is the standard for size
http://www.minibull.org/general_appearance.htm
You should have AKC papers (or the kennel club for your country if not in the US) for a miniature bull terrier and have seen one or more of the parents. They are a separate breed from the bull terrier and are not interbred with the larger breed.
Adult miniature bull terriers tend to weigh in at 20 to 35 pounds so if you double the weight now and get more than that your pup could end up being oversized for the breed. This does not mean it is not a purebred. In any small breed there can be throwbacks to larger sizes. For example I've seen pomeranians which are supposed to be 3-7 pounds weighing 35 pounds like some of their ancestors did.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics