Ok, think of the prop as an extension of the gears.
A bigfoot already has "low gears" compared to a (regular) outboard, but they are "fixed" at 2.3:1 (a regular 60 would be about 1.8:1).
Since that particular motor could be used on a variety of boats, the second half of the equasion is the propeller diameter and pitch.
If that motor is pushing a relatively light vessel that has good hydrodynamics it would probably have a 13 or 14" diameter, 13 or 15 pitch propeller.
The more weight the motor is asked to push, the harder it has to work. If it's working too hard (rpms not getting into the prescribed range) you have to lower the pitch (and possibly the diameter) in order to lessen the load on the motor.
Basic rule of thumb - for each 1" decrease in pitch, your rpms will rise about 200 and each 1/2" decrease in diameter will also raise your rpms by about 200.
So if you are topping out at 3500 rpms, and you should be (at minimum) in the 5500 range that MAY mean that your prop is about 10" of pitch more than that motor can effectively turn given how much weight you are asking it to push.
If your current prop is a 14D (diameter), 15P (pitch) changing that to a 13D 7P should allow the motor to recover the 2000 rpms that it's currently losing
1" less diameter = +400 rpms
8" less pitch = +1600 rpms
But as I stated above, I would ensure the motor is properly tuned first (before trying to determine the correct prop). You could be losing a good few rpms if it's been neglected...