Quote from profitseer:
average es size is 1.5 (this posted by a trader on this site who sounded like he had the actual number from the merc)
Keep in mind there is a difference between trade size and ticket size. For example, if I bid 10 lots and 10 different people offer a 1 lot and sell a single contract to me, then the average size of the 10 trades is 1 but the average of the 11 tickets is 1.8.