In most instances this can be found in the manual, if for some reason it is not listed, then going bigger is a better option. If you have a 13.5 amp snow blower, then typically you can get away with a 100' 14 gauge cord, but it would not be bad to use a 12 gauge and by no means use a 16 gauge at the 100' length. Most of the snow blowers can actually handle a 16 gauge at 50', but I don't usually recommend it because those motors are not what I would extremely high end. If you got into a heavy load of snow for some reason the snow blower is going to stress as is so you need the most consistent amount of power running to it.

I will throw out an example to you. With our Toro 15 amp unit, Toro is going to recommend using a 14 gauge for 100'. It then suggests 12' gauge for 150'. This can vary, but I think if you are going with a 100' cord, you should be fine with 14 gauge, unless you really wanted a 12.