Keep these rules of thumb in mind when selecting crown for your house.
Consider the room's purpose. Traditionally, crown was incorporated into impressive cornices in public rooms, such as entry foyers, dining rooms, and parlors. The size would remain consistent from one first-floor entertaining space to the next. By contrast, smaller, less elaborate crown was generally used for private spaces—a powder room or bedroom.
Play off the existing trim. Look to your baseboards and door and window casing to determine the right scale. An appropriately sized cornice is either the same height as, or slightly smaller than, the baseboard.
Measure the ceiling height. Ceiling height is the ultimate arbiter of size, according to architect Richard Sammons, coauthor of Get Your House Right. For standard 8-foot-high ceilings, the crown/cornice height should be 2½ to 6 inches; for 9-foot ceilings, 3 to 7½ inches, and for 10-foot ceilings, 3½ to 8 inches.