In the Powers of Ten showcase, the closest power of ten to "human size" is the zero power of ten, because it is one meter. (See the label below the picture of the oak tree branch with leaves.) Each decrease by a power of ten means a reduction to one-tenth of the previous size. So, a decrease by two powers of ten reduces something to one-hundredth of the original size; a decrease by three powers of ten reduces something to one-thousandth of the original size; etc.