A shepherd/terrier mix's adult size varies based on the animal's health and genetic history. Some adult shepherd breeds weigh in excess of 150 pounds, while terriers are smaller animals. A mix of these breeds results in dogs of varying sizes.
According to ScienceDaily, a dog's genome contains six or seven locations that account for the majority of the dog's physical appearance. Similar to humans and thousands of other species, the offspring of two individuals results in a unique mix of the two parents. Some dog mixes resemble one parent more closely than another, but it is impossible to project the adult size of an animal of mixed origins.