There are many breeds larger than a "Bullmastiff" (correct
spelling). Mastiff, etc.
Read up and study this breed if you are interested in it. There
are many books out there on Bullmastiffs. You want a dog that is
within the standard as set by the AKC. Not just a BIG bullmastiff.
There are standards set for height, weight, etc but you need to
study this large beautiful breed, they are not for everyone.