I am sure you know that no respectable historian today believes the million men-plus number Herodotus gives for Xerxes' invasion force!
It probably numbered no more than 200,000 at most.
To my knowledge, the largest army ever assembled in antiquity was the "million-men" army that the Sui tyrant Yangdi fetched against Koguryo, a powerful Korean-Manchurian kingdom. In this case, while contemporary historians do not accept the one-million number, they do assume that this army may have perhaps number over half a million--which makes it around the size of Napoleon's army when he invaded Russia. The Sui numbers are far more reliable than the Persian numbers, because 1) the Sui number was given by firsthand (that is, Chinese) sources, while the Persian number was given by the Greeks; and 2) the Sui kept a far more detailed and therefore credible record of the composition and organization of their massive army than the Greeks writing about the Persians.