A face cord of firewood is 4 feet tall and 8 feet wide, same as a full cord of wood, but the depth is only one log deep (cut logs are approximately 16 inches long).
A full cord of firewood is just three face cords deep so it ends up being approximately 48″ or 4 feet deep.
A face cord is a great size if you plan on building a fire only on days when the high temperature is 55 F or below.
A full cord will last longer if you want to burn firewood several times a week in a firepit and/or fireplace.