Like others have pointed out - it depends on your local zoning laws, region, type of grass, how much you plan to pick the area clean, how much hay you want to feed, etc.
I've found with horses - smaller are per animal actually ends up being more work because there needs to be more care in keeping the area well picked out, grass maintenance, hay feeding, etc.
We have about 1/2 acre dedicated to the 3 minis (1 donkey and 2 mini mares). We have their main area that is about 80X80 feet. This area has some grass growing but a lot of it is "dry lot" type of ground. It gets mowed for weed control more than anything. Then off this we have the rest of their 1/2 acre that they are let out on for a few hours a day. This area has had to be mowed several times this summer to keep the grass down because they could not keep up.
But that is here in KY. In other areas with different soil and grass that area may not be worth anything more than exercise room and can't maintain enough grass. It all depends.