Actually, since the 101# runs on 36 volts, it draws less amps (46)than many 12 and 24 volt motors. 6 gauge will be fine. If you want to run the math, at the bottom of this page is a calculator that will let you run various wire sizes and different lengths. Although yours is a 36volt, I tried it at both 24 & 48 volt selections, and the voltage drop stayed the same. http://www.powerstream.com/Wire_Size.htm HRG
I have 4 gauge in my 18 foot boat and 6 in my 16 foot boat. Since you are running a 101 lb trolling motor no matter the length I would opt for the 4 gauge. Make sure you use a marine grade wire it has a more dense insulation that can stand up to the vibration and sudden jars associated with running through waves.