Labs, Goldens, etc are large breed. Great Dane, Mastiffs are classified as Giant Breed. It goes by weight usually, then depends kind of who you ask. Like AKC vs UKC etc, however medium breed say 25-50lbs Some have it 30 lbs as minimum, some have 60 has maximum but all around that area. Large breed are going to be 50-100 lbs again some say min is wherever their medium leaves off so could be anywhere between 50-60, and maximums are 90-110 lbs, anything over the maximum is going to be giant breed.
This is based kind of on the breed average weight as you can have larger and smaller dogs of the same breed as I have known labs that were not overweight fat but large in the 130lb range and smaller labs in the 50lb range, but most labs run in the large breed weight class.