In order to know the limits of the size of the APBT you would have to go back to the original practice of the Pit. Competitor would inspect the opponents dogs by picking them up and checking them. So the weight of the dogs were restricted to a narrow range.  Even in the ADBA standard I think the upper limit of 75 pounds is too much.

I will post something else on this later when I have a chance to refer to my old books and articles.