Male bullmastiffs grow to be 25 to 27 inches tall (63.5 to 68.5 cm) at the withers and 110 to 150 pounds (50 to 60 kg). Females typically reach 24 to 26 inches (61 to 66 cm) at the withers, and 100 to 120 pounds (45 to 55 kg).
Any shade of brindle, fawn, or red is allowed as long as the colour is pure and clear. In the United States, however, there is no mention in the standard of the colour being "pure and clear". The fawn is a light tan or blond colour, while the red is a richer, red-brown. This can range from a deep red to a light red merging with the fawn sometimes described as a red-fawn. A slight white marking on the chest is permissible, but other white markings are undesirable. A black muzzle is essential, toning off towards the eyes, with dark markings around eyes contributing to the expression.
The Bullmastiff is courageous, loyal, calm, and loving with those it knows. It has a very strong protective instinct and will defend its owners against anything it perceives as a threat. However, it does not normally attack to protect. Instead, it knocks the intruder over with its massive size and pins them to the ground, or, will simply stand in front of the stranger/intruder and refuse to let them pass. Bullmastiffs become intensely attached to their families and do best when they can live inside with them. Their protective instinct combined with their great size and natural wariness of strangers means that early socialization is a must. The Bullmastiff may or may not get along well with other dogs. Occasionally, females in heat will also not get along with other females. The Bullmastiff gets along well with children and is very loving towards them. Parental supervision should be maintained when they are with children because these dogs are so big that they may accidentally knock smaller children down.
Bred by English Gamekeepers in the 1800s to tackle poachers, the Bullmastiff (also known as a Gamekeeper's Night Dog) was a cross of 40% English Bulldog for its agility and tenacity (which was the Old English Bulldog, not the short, fat Bulldog of today) and 60% English Mastiff for its size, strength and loyalty. They bark much less often than other breeds, but when they bark they will make your head turn, as it is dark and hollow sounding. The Bullmastiff was recognized as a pure-bred dog in 1924 by the English Kennel Club. In October, 1933, The American Kennel Club recognized the Bullmastiff.Source: wikipedia.org