Sizes of boxes range from about 5" to as much as 20" in the length of one or both of the sides. Smaller boxes prevent down from lofting to its full potential and not trapping as much air. This proves to be less beneficial for insulating purposes. Larger may boxes may not be adequate either as they allow the down to shift within the box, perhaps leaving areas without insulation. Ideal size boxes range from about 150 - 225 sq. in. or 12" x 12" to 15" x 15".
Box Construction
Quilts (aka duvets or doonas) are created with different types of construction, including box and channel. Box construction usually divides the quilt into equal size squares, trapping down and/or feather in each of the boxes, preventing it from moving from one area to another. Sizes of squares vary depending on the particular manufacturer.
Box Construction is preferred to channel as it keeps the down and/or feather evenly distributed throughout the quilt. Without this, the quilt would have areas with little or no fill, leaving an area without insulation for heat to escape.