Typically, a twin bed comforter measures 66 to 68 inches wide and 86 to 88 inches long. The exact size varies by manufacturer. A comforter with these dimensions fits a standard twin-size mattress that is 39 inches by 75 inches.
When purchasing a twin bed comforter, a consumer must also consider the depth of the mattress. While a traditional mattress is 9 to 12 inches deep, modern pillow-top mattresses run 14 to 16 inches deep or more. These require comforters that are larger than the average size. In addition, fluffy comforters, such as those containing down, are often quite puffy and do not spread out to their full dimensions.