How do I determine the right propeller for my 26' pontoon boat with a 1998 mercury outboard 90 hp motor?

The propeller size is determined by the engines wide open throttle RPM

Let take an example of an18' boat with a 125 hp engine with a rpm range of 4500-5500. In most cases you purchase a boat with some sort of propeller on it. Let's say this motor has a 21 pitch prop on it and at wide open throttle under a normal boat load, the engine turns 4900 rpms. Well it's in the range so it's the correct prop, right? Well yes and no. For the health and longevity of the engine it's okay, but for overall performance, you want to run a propeller near the top of the range. An 1" of pitch in a prop can change the RPM 200-400. If you need more RPM decrease the pitch and vice versa for less. Remember this is a trial and error and not an exact science. Factors like loading the boat, fuel and water all have to be considered.