In any experiment, ou have to figure out what factors to fix and what are variable.
The variables here are your crank length, what happens when you alter it.
Do you want to fix gear ratios? Not neccessarily since the same rider on different cranks would select a different ratio. See Sheldon Browns Gain Ratio measure.
The pedalling force should remain constant but the circumference of a rotation will increase with crank length so you would expect cadence to drop and gear ratio to increase a notch.

Is power output a variable: No, your body has a fixed upper limit and a set crusing level. Bike speed is the variable here.

Fix your heartrate and pedalling force. Select gear ratio and cadence to suit and measure the resultant speed. Do this on a ~5 mile time trial circuit in both directions.