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There is far more to propeller design than just diameter and pitch. Props are not generic pieces of hardware. Each model of propeller has certain characteristics that do well on some boats and not so well on others.
So when I see advice given in the form of: "Try a 13.25x17 or a 13.5x19" I know that is not knowledgeable advice.
We know from personal and anecdotal experience what some models of propellers do well on many different Whalers. In the case of your boat and motor, the choices are not extensive. I would recommend the 13-1/2" x 20" Mercury Laser II or the 13-1/4" x 19" Stiletto Advantage I. There are other props one could try but those are the two most common.
The reason diameter does not get mentioned too much is that within any given line of prop, the diameter is set by the designers for each pitch offered. You choose a prop from a line of props based on the pitch that will fit your boat/motor. You do not decide the diameter, nor do you pick a diameter and then take whatever pitch it comes in.
Diameter *is* discussed when talking about the different models of propellers. For example, on large outboards, there are many, many prop choices and one might argue the relative merits of the large diameters of the MIRAGEplus line versus the smaller diameters of the Laser II line. None of that discussion would be applicable to your Mercury 90.
