It doesn't matter how much land it takes to create animal protein, not per se, not in relation to sustainability.
The Great Plains once has giant herds of bison roaming across them. Humans could eat those bison sustainably as long as they didn't take enough bison to disturb the equilibrium between bison and grass. Taking one bison out of the equation would simply cause the equilibrium to produce one more bison. Reducing the buffalo herd from 25 million to 600 on the other hand is a different matter.
What matters for sustainability is the disruption of natural systems, not the acreage.