The Torah commands one to make a blessing after eating bread and defines 'eating' as constituting the ingestion of a 'k'zayit' (the amount of an olive). The requirement for making a brocha achrona on other types of foods uses similar criteria. To know how much of a particular food equals a k'zayit is not easy. A k'zayit is a measure of volume, or the amount of space it occupies. But Rabbi Yisroel Pinchos Bodner of Lakewood, New Jersey has researched this and come up with the answers in his book, "Halachos of K'zayis". According to Rabbi Bodner a k'zayit of herring is equal to two large pieces equaling about 29 grams and looking like they would fit onto two regular sized crackers.