My 90w low pressure sodium streetlight is about 3 feet long (but still weighs about as much as any other streetlight). Do I need a larger or thicker mounting arm for it compared to a regular streetlight that is about half the length?

 I wouldn't think so as most of the weight is right by the slipfitter . Also , a Lot of the larger cobras were WAY heavier and longer than the SOX lights with their heavy refractors far to the end of the fixture .

 Examples :  1)  My 700w MV OV-50 weighs 70 lbs. and is almost 40 inches long . The refractor alone is about 10 lbs.
             2)  My Finned M-1000 weighs 58 lbs. and is almost 36 inches long . The refractor alone is about 1/4 of the weight.
             3)  My 90w SOX fixture is only 30 lbs. and about 80% of the weight is located in the first 10" of the light .

I would be kind of worried about it blowing in the wind if it only weighs about as much as a small size cobra, but is almost twice as long.