Minecraft wiki entry for Spawner, associated text: 8×3×8 is the actual volume across which spawners spawn entities, but this volume contains only the entity's center. The failure rate assumes 8×4×8 because it is a popular misconception that 8×4×8 is the maximum efficiency volume of empty space for mob spawners - it actually depends on the size of the entities themselves. Note that 10×4×10 is guaranteed to accommodate all of the above mobs. Also note that this failure rate accounts only for failures due to attempting to spawn a mob at the edges of the spawn range, not other causes of failure such as mobs attempting to spawn intersecting the spawner block itself.