the reason that they come in different sizes is for the different size spindles on your cranks.


Profile cranks have 19mm spindles...
and Primo cranks have 22mm spindles...



If you dont have either of those cranks you may want to search the internet for your size spindle or have a bike shop check for you...

Its not the size of the bearing itself, its the size of the hole that your spindle fits in...






good luck.