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".