Hi Nigel,
I've no experience with the model, but Sam is about right:
1. get a watt meter and measure the current at full throttle
2. work out what the maximum current is for the motor (quoted value, Hackers are usually happy at the rated value) and the ESC (I'd use no more than 90% of rated value); the lowest of these sets the maximum current for your system.
3. Increase pitch or diameter until you get to the maximum current.
Whether a 6s set-up is enough is partly a matter of opinion:
1. If you increase the current you may need to run higher C rating batteries. For example, a typical 2m F3A setup uses 5000mAH 25C batteries with a maximum current of around 75A. 75A is 15C, so only about 60% of the rated value.
2. In general, if your require large currents to get the necessary power (say >80A) I would suggest that an 8S setup would be better.