The diameter of the street light poles depends on the height, the installation place, and the weight of the lamps. Roughly speaking, we can decide the diameter according to the height. The bottom diameter of 7-meter street light poles can be between 140 and 145 mm, while that of 8-meter poles can be 160 and 165 mm. And every country has its own regulations about the smallest value of the diameter.
More reading: Height of Street Lights: How Tall Should They Be?