Watts = (voltage squared) / Resistance
As mentioned above, 6 watts is a good place to start. Your eGo Twist has a pretty wide range, but will top out if you try to put a low resistance on and vape at high voltage--it simply can't produce any more power, so turning up the dial does nothing.
Most people seem to like about 2.5 ohms, plus or minus, which would be functional through most of the range.
And for example, Watts = 4^2 / 2.5 = 16 / 2.5 = 6.4 watts, or very near the sweet spot (for most people). Assuming you've set the Twist to 4 volts, of course...
With the Twist, you can play with the voltage to your heart's content, so any reasonable resistance will do--from 1.5 to 3. I'm not sure what the lowest allowable resistance is, but it's usually safe to assume 1.5 and very few commercially-available, pre-coiled atomizers go below that.