What is the best pole diameter for beginners?

It depends.

If you have aspirations to compete or entertain in pole dancing competitions or shows then a 45mm dance pole is your choice, its the industrial standard.

If you just want to utilize pole dance as a fitness tool for your own well being then customizing it’s size to you is more valuable.   If you are a short, small person with small hands, get a 40mm dance pole or 42mm dance pole.   It will be easier to grip in your training.   If you feel you are average with medium or large hands, shoot for a 45mm.    My apologies, there is no size chart for measuring this in the dance pole industry yet.

50mm dance poles are an industrial standard set by stripper clubs that have very little weight in determining whats right or wrong for the average person.   (no pun intended)

If you do plan to become a stripper as a profession then choose 50mm.   Its the standard size in strip clubs for decades.  While almost every major dance pole manufacturer has stopped producing this size, you can still find it at Lil Mynx.