For a comb-over haircut, hair should be 2 to 4 inches long for a traditional look. For a modern style, opt for more volume on top. When styling, ask your barber for a hard or natural part and specify the desired look for the sides and back. Choose a guard size between 4-6 to get the best result, and pair it with a fade, undercut, or hard part. For a quick and versatile look, go for a short comb-over with hair cut to 2 inches and tapering at the sides and back.