Hi, I am Dr. Jeff. I will try to help you. Please feel free to follow up if any more information is needed. Most dogs' growth
plates will close around 9 months to a year, which ends bone
lengthening. Keep in mind, some dogs will still fill out in their muscle
bulk (especially these giant breeds), but do not mistake this for allowing then to gain excess fat.
With St. Bernards
, there can be a wide range of size and weights at this age depending on the individual. I have some that reach an adult weight of 90 pounds, which I also see some close to 200 pounds. the key to a healthy weight is making sure he continues to have a waist. Many owners will "aim" for the parents weight. Keep in mind, this may not be the ideal weight for Rocky, so use your veterinarians guidance as he continues to develop.
In the end, the mamjority of the 6 month old St Bernards that I see range from 60-100 pounds.
Once again, keeping a good body condition in this breed is paramount for a long, happy life as it lightens the load on the skeletal system as well as your wallet (keep in mind, heavier dogs equals higher drug dosages).
I hope this helps and gives you direction.