But to clear things up for you. There are two main dimensions with props, diameter and pitch. Diameter is self explanatory, draw a circle touching the outer edges of your blades, thers your diameter . Pitch can be described as measuring the corkscrew --the theoretical amount the propeller would travel forward in one complete revolution. More pitch, the further the boat will travel per revolution, so the engine has to work harder. So you need to work with both diameter and pitch. But different designs will give different results for for identical pitch and diameter. And there is a lot to consider, but with a lower HP motor such as yours, not as much as the bigger rigs.

Now, can anyone tell him what he actually needs?

https://www.boats.net/catalog/yamaha/outboard-by-hp/70hp/70tlr-0408/lower-casing-drive-2