Minecraft wiki entry for Advancement_guide, associated text: First, you must prepare a 4x4 area then place fences all around the perimeter. This area is where the mobs will be trapped in. For this method you must also use passive mobs such cows or sheep. With these kinds of mobs, this size should be sufficient to trap all five mobs inside without any issues. If you are using horses, llamas, or any other passive mob that has a larger size than a pig, cow, or the other normal-sized mobs, it is a good idea to make the fence area slightly larger.