The length of the flag should be between one-quarter and one-third the height of the flagpole. Example: using the 1/4 rule, a 20 ft. flagpole would use a 3x5 ft. flag. The two major factors for knowing which flag you should use for your in-ground flagpole is the material of the flag and exposure to wind.
Materials for flags are Cotton, 1-ply Polyester, 2-ply Polyester, and Nylon. Cotton fabric is for the purest at heart, it is very durable, but fades fast under extreme sun exposure. 1-ply Polyester fills the needs of the budget minded. And the most popular fabric is Nylon because it is moderately priced, takes a light wind to lift the flag and is fairly durable. But if your flag is wearing out every 6 months or less you will need the tough 2-ply Polyester, it is made for persons who fly their flags in high wind areas or extreme outdoor conditions. For more questions regarding the durability of outdoor flags, please see our How long will a flag last? article.
To determine the amount of wind your flag can be exposed to see our Wind Speed Chart. This wind speed chart is for the United States of America only. Of course all flag flying is unique to its location and surroundings, i.e. wind tunnels, ocean breezes, and landscapes.