Tonsil stones are the reason more than 25% of the population suffers from chronic bad breath! They occur when sulfur-producing bacteria and debris collects within the mouth.
They pose no problem except bad breath and thus are never suspected to be present.
Tonsil stones are mostly small enough to not be seen by naked eye, also their position is such that personal inspection is very hard.
Proper brushing and use of oxygenated mouthwash and dental floss. That is all you need to do to prevent them. Even removing them is no rocket science. Just like ears, a clean cotton swab can be used, only moisten it beforehand and then gently massage it out.
That is how big they can get, even bigger. Although it is best that the doctors be consulted if regular occurrence is observed.