What is the difference between a small amount of impurity and a large amount, how is that estimated, and what is the amount of a small amount of impurity?

The small amount of impurity that is excused is defined by some scholars as that which people ordinarily overlook, or that which does not seem excessive to one's heart, thus its estimation is left to the person afflicted by it. If there is doubt whether it is little or much, it is treated as a small amount.