Several companies publish charts to properly size wire to use with submersible motors. With pipe size it is rarely a problem to use larger pipe than needed. However, with wire size, it can be beneficial to use the longest length of the smallest wire size possible. This can eliminate the need for a soft starter with submersible motors.
Quote from Franklin AIM manual:
"Reduced-voltage starters may not be required if the maximum recommended cable length is used. With maximum recommended cable length there is a 5% voltage drop in the cable at running amps, resulting in about 20% reduction in starting current, and about 36% reduction in starting torque compared to having rated voltage at the motor. This may be enough reduction in starting current so that reduced-voltage starters are not required."
Reducing the wire size will not only save money on installation cost but, can also function as a soft starter.