Step motors are categorized by frame size, such as "size 11" or "size 23". Ever wonder how that came to be or what it means? The National Electrical Manufacturers Association sets standards for many electrical products, including step motors. Generally speaking, "size 11" mean the mounting face of the motor is 1.1 inches square. So a Size 23 step motor is, wait for it, 2.3 inches square. Or 56.4 mm as shown below.
This photo includes NEMA size 34, 23, 24, 17, 11 and 08 step motors. Size 24 is somewhat of an outlier - there is no formal NEMA definition for size 24. It mounts like a size 23, but is slightly wider (about 4 mm), allowing for a larger rotor and stator and thus about 30% more torque.
NEMA standards help provide interchangeability - the ability to switch from one manufacturer to another without having to significantly change mounting brackets, couplings, etc. I'm sorry, did I say "significantly"? Indeed, two motors of the same NEMA size may actually differ somewhat among manufacturers. Shaft length is one examples, and the presence of a flat for use with set screws varies among vendors. NEMA standards also do not dictate the number of lead wires or winding impedance, so be sure to consider all specifications carefully before designing in an alternate supplier.