Simply measure from the top of your mounting bracket to the bottom of the boat transom, this will give you the shaft length needed.
Note: your cavitation plate over the propeller should be even or below the boat transom.
With a Troll master Pro with Panther steering you should be set to start cleaning fish.
Not sure about the configuration of your boat but you will need to ensure the bracket can handle the weight of kicker, the closer you are to the boat the more weight and vibration the bracket can handle,it's that old leverage thing! Bob