For a 20-foot residential flagpole, the most common and well-proportioned flag size is 3' × 5'.
- Best overall: 3' × 5'
- More prominent display: 4' × 6'
- Heavy or windy location: 3' × 5' is usually the better choice because it puts less strain on the pole and hardware.
- Commercial or larger-diameter pole: A 4' × 6' may be appropriate.
For most 20-foot flagpoles, I would recommend a 3' × 5' nylon flag.