He wouldn't be as large as a mastiff because of the other breeds in him. That's the joy of a mixed breed, you never really know what you're going to get in terms of size, temperament, or looks. It's anyone's guess. :)
You have a beautiful dog, he looks healthy and well cared for, that's really all that matters in the long run.
I have a border collie mix that acts nothing like a border collie. When you look at him the border collie is the breed that stands out the most, the other breeds involved in his makeup are pretty well hidden. He's lazy (not a border collie trait), tall (for a border collie), and really doesn't like to herd all that much, but when people see him they assume he's a border collie. Thankfully he is smart like a border collie, so that's a plus. :)
Then there's my border collie lab mix. His father was purebred border collie, his mother was border collie/lab, but he looks nothing like a border collie. His siblings do, but he's more like a black lab. The only thing even close to border collie about him is his tail and his head. Not the look of his head, but the size. His head is too small for his body because of the mix. His fur is very soft and a bit longer, unlike a lab, but other then that he's more lab then border collie, even though the border collie really should have been more prominent.
Either way, you have a beautiful dog. Congrats. :)