הלכה על חלה 2:7

The measurement [to be given] for challah is not [specified in] the Torah - and even if one separated only a barley grain from the kneading, he would exempt the whole kneading in the trough from the law of challah; as the Torah only says, "The first of your kneading, set aside challah." [Thus] anything that he sets aside [suffices] to exempt him. The Sages however obligated us (Mishnah Challah 2:7) to separate 1/24th, which they based upon what the verse stated about this commandment, "give to the Lord," meaning that the gift should be dignified, and this was [the amount] that they estimated. A baker who prepares his [dough] for sale in the market - and [the dough] is large, and he also needs to make a profit - was only obligated to give 1/48th. And whether it should happen that the professional prepares a small [amount] or the homeowner prepares a large [amount], their [laws] are [still] as we said above.