Diet and exercise will not play a role in adult size near as much as genetics. You really have to know the line of the pup to even come close. Even then your pup could end up the smaller of the line, or the largest. Growth rates won't really tell you much either. You could have an early bloomer, or late bloomer. The closet I've seen to accurate estimate of adult size is double their weight at 12 weeks, and add 1 lb. Or double 14 week old weight. That should get you fairly close.
Diet and exercise is very important. But your pups frame will not be considerably effected by it. It will determine a healthy, or unhealthy weight for their frame. 9 to 12 months of age, your pups growth plates will be closed, and his/her frame (height/length) complete. Significant growth is slowed around 6 months of age.