The United States has no standard size for a single shot. The standard size for a shot is generally considered to be between 1.25 and 1.5 fluid ounces. Short shots are usually 1 fluid ounce, while a double shot is usually 3 fluid ounces.
Typically, a shot glass is made of thicker glass, especially the base, to prevent it from breaking when the glass is slammed onto the bar. The state of Utah is the only part of the United States to define the size of a shot. Utah defines a shot as 1.5 fluid ounces. Many other countries have varying standard definitions of shot sizes.