The following site goes over the usage, precautions and dosage information rforpenicillin G.
I have to advise you that no medication should be given without first notifying your own vet. In addition, if your dog's mouth is swollen, then there may be an abscess forming that needs to be drained as well. If the dogs were fighting, your dog might also have injured a disc in the back leading to problems with the rear legs. So I do recommend seeing a vet if possible. Hope this helps.
I am trying to find the dose for the dog. This site was not helpful. Do you know the dose? if not I will try someone else.
The site does give the dosage as 10,000 to 25,000 units per pound 2-6 times a day. My vet would probably go midrange at about 15-20,000 untits per pouind three times a day. Antibiotics are normally given for 10 days.