Minecraft wiki entry for Huge_fungus_generation, associated text: To determine the height of a huge fungus, a random integer is picked between 4 and 13 [inclusive], then there is a 1/12 chance for that integer to be doubled. This integer is the height of the trunk in stem blocks, meaning that the tallest tree can be 27 blocks tall (as the top is covered in hat blocks) with a 1/120 chance of generating.