The 6091 does not tune wave size, it tunes the frequency and only one frequency will create a wave. It can be just a hair width increment from a 1/2" -14" wave, but it will be a very particular frequency that is termed the resonance frequency that creates an actual wave. Once you find the wave frequency, wave height can be adjusted by the pump power knob which is the small white or red knob on the connector. I would start with the pump at full power, and search from full counterclockwise to about 2 o clock in hair width increments turning it just a small amount every few seconds. When the wave is always the same height and doesn't build and diminish over time, that is when you have the frequency spot on.
What corals are you keeping?