Those are giant dogs! But at least in the US they are vanishingly rare. Additionally, the size of the dog doesn’t necessarily correlate with equipment size, just as an FYI.
Just like anything anal, lots of practice and lots of relaxation go a long way, as well as committing 100% so you get the knot further in, where there’s more stretch. “Too big” really depends on how fast you’re trying to scale up, because dogs will enthusiastically not take things slow.