Sifrei Mitzvot para sa Levitico 19:10

How much is the [minimum] measure of an "edge" portion? From the Torah there is no [minimum] measurement. Even if one left one stalk, one is free from one's obligation, but from rabbinic law it is not less than one-sixtieth whether in the Land of Israel or outside, and one increases over and above one-sixtieth according to the size of the field, the number of poor people, and the blessing of sowing. How so? If a field is exceedingly small and one leaves from it one-sixtieth, it does not benefit a poor person. Thus in this case the measure is increased. And so if there are a great deal of poor people one adds [to the measure], and if the sowing is very little and nevertheless one took in a great deal because one was [unusually] blessed, one increases what one gives according to the abundance of the blessing. Anyone who adds to the measure of the péah portion increases his reward [by God], and there is no [maximum] measure to this added benefit.¹⁶See Mishnah Péah 1:1-2. The first line of Mishnah Péah reads, "These are the things for which there is no definite quantity: péah, first fruits, the festival offering upon appearing in the Temple, deeds of lovingkindness, and the study of Torah." Taken literally, this means that the Torah does not prescribe a limit in the application of this duty. The implication, however, is that there is no prescribed definite quantity for the reward of following these commandments, as Maimonides makes clear in this law. In addition, one is supposed to be as generous as possible.

To leave the harvest of the corner of the field or orchard for the poor (Lev. 19:9).

He prohibited - not to gather the single grapes that fall in the vineyard at the time of the harvest, but one must rather leave them for the poor. And that is His saying, "neither shall you gather the single grapes of your vineyard, etc." (Leviticus 19:10). And this too is rectified by a positive commandment; and the regulations of this commandment have already been explained in Tractate Peah. (See Parashat Kedoshim; Mishneh Torah, Gifts to the Poor 1.)