Will the length of my dog’s hair keep them cooler/warmer during the different seasons?

In most cases, no. Dogs are built for weather changes. Some smaller dogs may still require something to keep them warm (like a sweater or such) in the cold winter and big dogs may need extra water or shade during a hot summer. But for the most part as long as there is no matting, the length of their hair has no effect on keeping them warm/cold.