The appropriate size flagpole for your application depends on how you want to present your flag. There is no right or wrong answer to this question; it is a matter of taste. Often times you want the flagpole to be the focal point of the landscape – then you will want it as large as possible. Many times you will want the flagpole to blend in with the structure and the landscape. You also want to take into consideration the wind conditions in your area, make sure the flagpole you select will stand up to the winds that your area is subject to.
The key thing to remember is that you want to stay away from power lines, trees, other flagpoles, buildings and any other structure that may interfere with the flag. You not only want to take into consideration the height of the pole, but also the size of the flag you plan on flying. Also be sure that you will be able to fly the flag without hitting any structure when flying the flag at half-staff.
A quick and inexpensive way to get a visual feel for the height of the pole is to take a kite string and attach it to a helium filled balloon. Raise the balloon in the area you were considering flying the flag, and measure how much string is out when you get it to the desired height.