Answer:
If the mother is the Old English Mastiff then the puppies would be bigger than if the American Bulldog was the mother.
If the English Mastiff was the mother then I would say that they could be almost as big or as big as the Old English Mastiff, depending on whose genes they have, probably lighter in the body, but very muscular.
If the mother is the American Bulldog, the puppies would probably be just a bit bigger than that, but probably not as big as an Old English Mastiff.
I have a Neopolitan Mastiff x Labrador bitch. Her mother was the labrador and she is about the size of a large labrador male, with a very deep chest but has her mothers labrador head and black labrador colouring, even though her mother was a yellow labrador. Her father was a brindle.