It's very difficult to say if a dog of a given size would survive a bite to the back of the neck and a slash on both the forelegs. Given that dogs have loosely-attached skin, a toy-sized dog might survive this... or a giant dog might have died.
However, there is another standard by which this situation can be judged, and that is the standard of the mindset of a solitary predator. These animals rely upon their health to be able to hunt, and if they are injured, they may easily die from starvation if their injury prevents them from hunting successfully. So, for this reason, real (as opposed to Hollywood) solitary predators are cowardly by human standards. They can't afford not to be.
So, by this standard, a dog - which is a pack hunter, and need not be cowardly due to having pack-mates to help if it gets injured - is a serious threat to a liguar if the dog is big enough to cause an injury.
So, we don't need some huge, horse-sized monster of a dog, we just need something big, loud and toothy enough to give the liguar pause and make it wonder how many friends the dog has and when they'll arrive. If the cat manages to injure a lone dog, but fails to kill it, it would likely retreat as soon as the dog manages to inflict an injury in return. That means that the dog need only be big enough to do so.
So, my guess is that the dog could be half the linear dimensions of the cat, and 1/8th of the mass (because of the square-cube law) or even smaller, and still be a reasonably effective deterrent against these cats. The more dogs are present, the smaller they could be and still be an effective deterrent.
So, if the dog in question has a number of pack-mates, it need only be big enough to cause an injury.
So, if we have a cat that may be around 2 tons given its linear dimensions, a 250kg dog should be more than enough of a threat to make the cat consider other prey. If the dog has pack-mates, maybe as little as 2 or 3 100kg dogs might be effective.
We can speculate that the Voswuinian Mastiff would be a dog with a particularly large, long head, a large gape and long teeth so as to be able to inflict as damaging a bite for its body mass as possible, without being too big. Very large dog breeds tend to be slow and lumbering and have shorter lifespans, and they also have to be fed, so the lighter the better.