We are installing light poles at our church. They are wood, 6x6, have 5' cross arms at the top which extend 12" to the back and 4' at the front with the light fixture. (and of course there is a 45 support under the cross arm). We intend to bury them in concrete with sonotubes 5.5' - 6' deep. Our question is how wide should the sonotubes be and should they be flared at the bottom. We do get some wind here so we want to make triple sure they are properly supported.
That's more of an engineer's decision to determine wind loads on fixtures, but here's some advice:
Modern commonly available 6x6's are a bad bad bad idea. BAD. Every knot is a weak point, and will fail in the wind. Not MIGHT fail, they WILL fail. You need wooden light poles. They are usually locally available for the power companies, and they are just as available to you guys. Price is comparable to a 6x6, they are properly graded lumber, and properly tapered.
Keep in mind that making the wrong decision here could result in injury, or death. Make the wise decision.
Past that, tapering your footing isn't a bad idea, but it can be done simply. Install the Sonatube above ground only, let the concrete go wide to the undisturbed soil below grade.
Size of the Sonatube should allow at least 2" of concrete around the pole, so long as you go the depth you stated.
For subsurface installations, structural engineer prefers using non-wolmanized 6x6's and coating with Henry's roofing tar on all areas, including the ends. Then, encasing 6x6's in concrete. Sono-tubes at surface area only. Use #3 or #4 rebar for cage in footings.