The size of the pipe transition would somewhat depend on what else you add to the sump output. For example, if you are allowed to channel your gutters to this drain, then you'd probably want to transition to the max size as soon at those were introduced. otherwise, you wouldn't need to do anything until you get near the storm drain. Depending on how deep the line is, it is no different than your normal sewer...it should be below frost line, and you shouldn't have to worry. The pipe DOES need proper pitch (slope) to it. That slope depends somewhat on the diameter, but for less than (I think) 4", it should be 1/4" per foot. Larger pipe can get away with slightly less slope, but it should still have negative slope to the drain point. Since it is liquid at the house, you have proper pitch, and it is deep enough, the flowing water won't freeze.