There are some ways you can try out different diameters before making a decision. If you have access to a local commercial gym, they may have different types of pull-up bars throughout the facility. Try some different options and see what works best between thin, standard, or thicker bars.  You could also look for a local store selling gym equipment and talk to an employee about testing some bars in different diameters.  If nothing else, you could always try buying one online, and you can likely return it to fitness equipment retailers if you get a pull-up bar and it isn’t the right fit.