pontoon prop size

[BP37066]

I have a 90HP Mercury 4 stroke and the boat is a 20 foot Bentley Pontoon. What do you think would be the best diameter and pitch prop to use?

 

[WesNewell]

12-15P depending on load and engine GR. What year model boat and engine?
https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one