But, did Herodotus see Xerxes Army first hand? Or was he just using Athenan propaganda ? It was big—real big, maybe 100,000 with an equal number of camp followers and support people.
Knowing the vast area and nations within the Persian Empire there is not much doubt they could have produced an army that size.
The main reason modern historians say it could not be that large was the logistics. Feeding and supplying that many on the march was probably impossible.