I was just wondering, would a 6x6 post of 16 foot make a good pole for a streetlight? Or a 20 foot post? I know Home Depot here have the 16 foot post, which it might be easier to move than the 20 foot.

Also I guess to set just dig a hole a certain percent of the pole length, and then have a few people lift the pole up and drop in hole?

The biggest problem is to do maintenance on the light, would be hard. Or do like sol did, and have the pole on pivot to lower for maintenance.

I used to have a 16 foot 4x4 post at my parents house in their backyard that I installed a bucket light on. I also had a small cobrahead on it at one point. We didn't calculate any percentage of the pole length but we did bury it a few feet maybe 3 or 4 feet. We also poured concrete around it in the hole. I had it close to the house so it could either be serviced by a tall ladder or by standing on the roof. A picture showing the upper portion of the pole is in my gallery on GoL.

I'm planning on using a 3" pole to convert my lanterns into floor lamps, and over here that seems to be the standard diameter for the upper portion of columns.  

Oops  our lanterns.

Mine, although it works well, is too heavy to lower when working alone. In hindsight, I would have made one of the metal channels half the pole height to better balance it. Having a Westinghouse MV Lifeguard lamp in there certainly helps with the lengthening of maintenance intervals.

I'm planning on using a 3" pole to convert my lanterns into floor lamps, and over here that seems to be the standard diameter for the upper portion of columns.  

Oops  our lanterns.

The post top lanterns here are less common I believe, but from what I can see 3" is the common size they fit. Our cobra and NEMA type lanterns for the most part use side arm mounting rather than post top.

We got a couple of brackets that fit onto the top of a 3" post to allow you to use a side entry lantern, though they're a bit bigger than we thought and will require modification before we use them.