The average daily volume of ZG seems to be about 20,000 contracts. Given about 21 hours of trading per day. 20000 contracts / 1260 minutes \~= 15 contracts / minute. So using a minimum of 100 contracts / bar chart seems reasonable. A 200 to 300 contracts bar seem to be a good starting point on first glimpse.
The key is to maintain reaction time of the system while not overtrading. I prefer to catch the big wave and not worry about the dips and spikes too much. Ideal holding period is in hours and not minutes.