There's no great answer for this.. except to measure them.
If you have any exsiting street banner, it's best to measure that. Ensure that you also measure the pole pocket size as well.
If you don't have an old street pole banner to measure, then you need to head up and measure the bracket spacing manually. If you brackets are fixed onto you lamp post, you MUST ensure a very accurate measurement. You should also measure the diameter of the bracket when you're up there as well. It is always better to make your street banners 1/2" - 1" bigger than needed in order to account for inconsistent installation of brackets, cold weather and material shrinkage as well as ease of installation.