2 sized smaller??? Cool!!! WTG!!!
When I was a store manager and had a similar issue, I wrote the person up and removed them from the register. Depending on how long she was working there, she may have been written up and retrained, moved to another position, or fired. If she's that bad at cashiering and is loosing money for the store, that isn't a good job for her. No matter what happens, it's not your fault, the responsibility for doing her job correctly lies with her, not with you.