Taanit, Chapter Three, Mishnah Six
This mishnah gives two concrete cases in which sages decreed fasts.
1) It once happened that elders went down from Jerusalem to their own cities and ordered a fast because there was seen in Ashkelon a shidafon which affected as much grain as would fill an oven [with loaves].
2) They also decreed a fast because wolves devoured two children on the other side of the Jordan.
a) Rabbi Yose says: not because they devoured [the children] but [merely] because they were seen.
Section one: A shidafon is a drying up of the crops, an event referred to in the previous mishnah. This story teaches that the amount of shidafon-effected crops necessary to justify decreeing a fast is enough grain to bake sufficient loaves to fill an oven. While I do not know exactly how much grain this is, it does not seem to be a particularly large amount.
Section two: This is another case addressed by yesterdays mishnaha plague of wild beasts. There is a debate about whether the wolves were merely seen or whether they actually devoured two children.