I just adopted a 8 month old male Newfoundland and Black Lab cross puppy. He is about 28 inches tall (from paw to shoulder) and 80 pounds. About how big will he be when he is full grown?

There is no way to say really. It's difficult to predict mid stages with a pure bred much less a mixed pup of two very different breeds. Definitely seems in that respect your pup is taking a bit more after the Newf in that aspect at least, lol, and is going to be huge. The most important thing is that they are healthy and a good weight for their own structure and body type.
They do a lot of 'up' growing the first year or so, then that begins to slow down a bit more and they do more of their 'out' growing faster and filling out and putting on their adult weight and muscle. AT only 8 months your pup has both ways to go still since he won't be really physically mature until around 2 to 3 years old.
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