The Federal Information Citzen Center provides information on recommended flagpole heights based on flag sizes.
Flag Presentation, Consumer Information Center: Our Flag
You may also want to check out this Web site:
Flag Rules and Regulations
This site has a chart which shows the appropriate size flag to fly on flagpoles of several heights.
You should also note that it states, "The Flag Code is intended as a guide to be followed on a purely voluntary basis to insure proper respect for the flag."