The National Association of Architectural Metal Manufacturers (NAAMM) provides criteria for determining the appropriate flag size based on the height of the flagpole.
If you know the flagpole height: Use this information to determine the maximum flag size suitable for the pole.
If you know the flag size: Use this to determine the required flagpole height.
This guidance is useful when selecting a new flagpole or flag and you have only one of these measurements.