The dog could be suffering from vaginal dropout and would need surgery to fix the issue. I work at and animal hospital and have seen only 1 case of vaginal dropout, but it seems to reflect the symptons you are seeing in your dog.
Depends on the size of the dog. A small dog about the size of a golf ball. A great dane about the size of a soft ball. Other size dogs vary with the size of the dog. Medium size about the size of a baseball.