Electrical faceplates, also called wallplates or coverplates, come in an infinite array of combinations, sizes, and shapes. However, there are some standard sizes for basic faceplates. Dimensions for faceplates and other electrical devices are covered by ANSI/NEMA WD 6, Wiring Devices - Dimensional Specifications.
There is a wide array of devices that can be accommodated by an electrical faceplate including: outlets, switches, motion sensors, telephone jacks, data jacks, dimmers, etc. Some of the more common devices are shown below.
Openings: 1.343" W x 1.125" H
Toggle or Switch
Opening: 0.401" W x 0.925" H
Decorative or Rocker
Opening: 1.310" W x 2.630" H
Telephone or Data
Gangs refer to the number of vertical groups of openings that are accommodated. The devices can change between gangs. For instance, in a 4 gang electrical box, one could have any combination of devices (1 toggle switch and 3 duplex outlets; or 2 toggle switches and 2 duplex outlets; or 1 toggle switch, 2 duplex outlets, and 1 tele/data; etc).
Electrical faceplates come in 1-gang up to 10-gang. Shown below are 1-gang through 4-gang
Shown: Single Duplex
Shown: Double Duplex
Shown: Triple Toggle
Shown: Quadruple Duplex
Standard Size Electrical Faceplates
All standard size faceplates are 4.490" in height. Widths are listed below. Please note that these sizes are standard; however, your faceplates may not match. Manufacturers make various sizes for non-standard conditions. For instance, there are oversize faceplates used to cover a larger hole in the wall.