The standard diameter of a US stop sign on a conventional road is 30 inches.
That measurement is taken across the opposite flats of the red octagon, not from corner to corner. The usual sign also has a white border and reflective white STOP legend so it remains visible at night.
The 30-inch version is the ordinary size used on most two-lane or conventional roads. Larger signs are permitted where traffic moves faster, where several lanes make the sign harder to see, or where extra reaction distance is needed. Smaller 24-inch signs can also be allowed in limited settings.
The octagonal shape is as important as the measurement: it lets drivers recognize a stop sign from the back, even before they can read the word. US standardization began in the 1920s, and the red-and-white design became nationally familiar after later editions of the Manual on Uniform Traffic Control Devices.