Size isn't the issue, there are 30 lbs street dogs in mexico that work as livestock guardians. It's just a mentality more than anything, but one a dogo definitely does not have. Livestock guarding is about having a subdued prey drive and an inclination to bond with livestock and be attentive and protective over them. Dogs that work with people and are evolved to be attached to people can't work as livestock guardians, and dogs that have heightened prey drives also can't work as livestock guardians.
Not just because "they might attack the livestock" (because this isn't true, you can break even a dogo argentino from attacking livestock- in fact most working dogos would be), it's more their interest in hunting will leave the livestock unattended. Whether it's hunting down the predators that come sniffing around, or random deer and hogs that pass through the property. They'll be roaming and hunting all the time.
Smart predators would actually just lure dogos away from the livestock and then have their buddies go get them. Livestock guardians need to be lazily disinterested in hunting and fighting and thoroughly bonded with the livestock like it is family. You also need primitive independent instincts to not be drawn to the human abode and away from the livestock.
So street dogs can work as livestock guardians, funnily enough, because they aren't bonded to people anyway, are just hanging around outside independently. They then organically become accustomed to the livestock that's milling about the village and know they aren't allowed to attack it lest they get a rock hurled at their face and get banished from the village or killed, and they're alert and on guard. Pariah street dogs naturally become livestock guardians by accident.
Weird primitive sighthounds like the azawakh can even work as a livestock guardian, at a stretch a GSD can kind of work as a shitty livestock guardian, but realistically you want real actual livestock guardians. Which are a related lineage of dog, they descend from street dogs that specialised for guarding livestock. There are actually lots of unseen adaptations they have made to be suited. Another one is they aren't very hungry, don't have big appetites, and can get by on a small amount of food for their size. A dogo argentino needs a HUGE amount of food, it's a high performance high octane machine and will visibly become emaciated and start starving to death in a few days. Then when it's hungry it's only going to be MORE likely to roam and hunt OR more likely to attack the livestock.
Basically we need to understand livestock guardians and gripping dogs are totally different animals down to the fibre of their being.
Their physical characteristics are very much secondary to their instincts, temperament and drives (or lack of drives).
