The consensus among poskim is that a prutah is 1/40 of a gram of silver, which is usually more than 1 cent but less than 5 cents. So if a person only gives a penny to tzedakah (let's say that is all that he has, or he wants to do the mitzvah many times) is it considered that he did a mitzvah or does giving tzedakah need to be with the lowest halachic amount of money, a perutah?
[See תוס' קידושין ג ע"ב ד"ה ואשה: "less than a worth of a perutah is not considered money" and many more where less than a peruta is not considered giving at all (regarding kiddushin)]