Difference between 38, 40, 42, 45 mm poles?

Hi guys, I have my lessons on a 42 mm pole but it seems hard to find a 42 mm pole that fits in my house.. So I was wondering: what is the big difference between the 38, 40, 42 en 45 mm? I do have small hands so I think 45 might be harder for me but since I only know 42 I would like to hear your opinions/experiences. Kind regards, Mieke