A cord of firewood is 128 cubic feet, meaning a stack 4 feet high by 4 feet wide by 8 feet long is exactly a cord.  A 1/2 cord is 64 cubic feet or a stack 4 feet wide by 4 feet high by 4 feet long.  A 1/3 cord is roughly 43 cubic feet or a stack 4 feet high by 8 feet long by 14-18 inches wide.