mini sizes (imp) start at 25'' then down. Avoid sizes 18'' down, they are micro minis, spin super fast and hard to transition your fingers/hands if your not use to minis.
I started with 24" personally, how ever if you have short arms like myself it makes doing fun tricks such as buzzsaws a little hard to do (watch out for your face!). I eventually moved to 20" and now I work with 18" primarily for focusing on tech.
when i started i found bigger ones easier to learn with, they go slower and are more obvious if they are off plane, and keep themselves more balanced. it also made it easier to visualize things for some reason. i started off with ones that went inside my arm but had trouble so i went to like 25-26" and things were much easier. if you go bigger, you want to make sure the tubing is light ( like 3/4 in on the outside ) or your hands will hurt. you can also buy a variety of kid hoops and take the beady things out, and then cover them with electrical tape to give them some weight and grip if you want to experiment with other sizes without spending lots of money.