Floor-length dresses are aptly named, as the traditional definition for the style states that the dress should just graze the floor. This implies that your shoes should not be visible when you are standing still. The length should be the same all the way around the dress.
Just Above the Floor
A more modern, practical take on the floor-length dress leaves the hem just above the floor. Some designers interpret this with as much as a couple of inches between the dress and the floor. At this length, the shoes will be barely visible, meaning the dress should be hemmed to a similar point on the shoes for bridal parties.
Consider where you'll be wearing the dress when deciding how close the dress's hem should be to the floor. If you'll be outdoors or walking up stairs, you may want the dress to be just above the floor. But if you'll be indoors, a full floor-length dress is the ultimate elegant statement.