I'd say you have two choices:
[ul]
[li]See if there's any standard light poles in the county, city or state standards. Find one that matches and use that.[/li]
[li]Go to IBC section 1805.7 (that's the 2003 IBC, maybe it's moved) and using guesswork, calculators and magic figure it out. This method has been used for about 80 years.[/li]
[/ul]