The most important quality of a duvet cover is its size. The correct duvet cover must be large enough to perfectly accommodate the duvet. The duvet cover should close tightly around the duvet without allowing much room for the duvet to move about within. Duvets are available in various conventional sizes depending on the type of bed for which they are intended, although their thickness might vary. In most cases, a duvet for a twin bed will measure around 68 inches wide × 86 inches long, while duvets for an extra-long twin should measure approximately 68 inches wide x 90 inches long. The dimensions of a duvet for a large bed can range from 78 inches long x 86 inches wide to 86 inches wide x 86 inches long. In general, the dimensions of duvets for the Queen and King-size mattresses are 90 inches wide x 90 inches long and 92 inches long × 104 inches wide, respectively.

A bed sheet is a rectangular piece of cloth used either singly or in a pair as bedding, which is larger in length and width than a mattress, and which is placed immediately above a mattress or bed, but below blankets and other bedding (such as comforters and bedspreads). A bottom sheet is laid above the mattress, and may be either a flat sheet or a fitted sheet (a sheet which has been sewn with elastic or a drawstring along the hem of the top, sides and bottom of the sheet to prevent it from slipping off of the mattress). A top sheet, in the many countries where they are used, is a flat sheet, which is placed above a bottom sheet and below other bedding.