The most you need to trade 1 ES is $ 500. That is the margin during normal business hours that my broker requires. However, I would suggest that you have at least $ 1,000 so you don't get a margin call. For anyone that suggests you need more than $ 1,000 to trade an ES contract, does not know how to trade. Period.
I know they do 25's without a hitch. I'm sure you can do odder than that, but your satisfaction with the outcome may depend on the liquidity of the instrument.