As titled says: How big is the range of Accelerator? Is it just on the block above it? Meaning just 1? Or does it have some kind of range? For example: If I set up 3x3 area and a growth accelerator under the middle farm block, will it affect entire 3x3 area or just that block? Also how many can I stack under until it becomes pointless?

Up to 64 blocks above it.