When selecting a king-size down comforter, one of the most critical factors to consider is the fill power, which indicates the quality and insulating ability of the down. Fill power is measured in cubic inches per ounce, with higher numbers signifying better insulation and fluffiness. A comforter with a fill power of 600 to 800 is generally considered high quality, providing warmth without excessive weight. Additionally, it is essential to assess the type of down used, as goose down typically offers superior warmth and durability compared to duck down..