The frame size of motors is used for both stepper motors and brushless DC motors. The length of the motor will vary but the frame size simply refers to the diameter of the motor face. Frame sizes are split up into NEMA (National Electronic Manufacturers Association) ratings which are a useful shorthand for motor sizes.

Don’t forget that within these frame sizes it is possible to have various motor lengths (sometimes known as the stack length). Changing the stack length will generally not impact on the speeds that you can get but it will have a major impact on the torque (turning force) that you are able to achieve. See the table below for an overview of frame sizes. Some such as NEMA 17 and NEMA 23 are very commonly used (these have been highlighted in orange). Others are generally available but less common.