I've found that the size is kind of like a ring size. I think if I remember correctly, because of a "writer's bump" on my finger I take a size bigger than my ring size. But since the regular thimbles seem to smother my finger I usually use a self-adhesive leather pad one. They're reusable for awhile, until they get to the point that they won't stick anymore. I've also used the clear plastic dots with success. And YES, it's trial and error until you find the one that works for you. Mom always prefered the old silver colored metal ones and those are the one I hate.