The National Association of Architectural Metal Manufacturers (NAAMM) publishes the following criteria for determining the maximum flag size for each flagpole height.
If the flagpole height is known, you can use that to determine flag size. Conversely, if the flag size is known, you can use that to determine flagpole height. This can be helpful when looking to buy a new flagpole or flag and only one of these two measurements is known.