Well, there is that one line in the Dog, Riding entry that should give a clue "This category includes working breeds such as collies, huskies, and St. Bernards" (Emphasis added). At least, that line is in the SRD.
The MM entries for domestic dogs can't individually cover everything in the wide variety of things we call "dogs". Grown Saint Bernards average a little over two feet in height, and something like 140 pounds. Great Danes tend towards a bit over 2.5 feet, but only average about 120 pounds.