Challah, Chapter Two, Mishnah 7. This mishnah does not deal with the amount of dough or the measure of flour that obligates one in separating challah, but rather with the measure of the challah itself - the amount that must be separated from the dough and given to the kohen.
The measure of challah:
The mishnah opens: "Shiur hachallah echad me'esrim ve'arba" - one twenty-fourth of the dough must be separated, made into challah and given to the kohen.
The basis for this measure lies in the wording of the verse: "From the first of your dough you shall give" - the giving must be in an amount fit to be considered a gift. The minimum measure that obligates one in challah is five quarters of a kav, that is, a kav and a quarter, and one twenty-fourth of that is considered a worthwhile gift for the kohen.
One who bakes for himself and one who bakes to sell in the market:
"Ha'oseh isah le'atzmo, veha'oseh lemishteh beno - echad me'esrim ve'arba" - whether one makes a small batch of dough for his own personal use or a large batch for a feast he is making for his son, the measure is the same. The Sages did not distinguish between a large amount and a small one.
"Nachtom shehu oseh limkor bashuk, vechen ishah shehi osah limkor bashuk - echad me'arba'im ushmonah" - here the measure is smaller, since those who bake for the market prepare a large quantity, and even one forty-eighth of it constitutes a significant gift.
The mishnah emphasizes that the determining factor is the intent to sell in the market, and not the status of the baker or the size of the dough: there is no difference between a professional baker who makes a very large quantity and a single woman who bakes to sell in the market - the very fact that it is made for the market is enough to set the measure at one forty-eighth instead of one twenty-fourth.
Dough that became tamei:
It became tamei unintentionally or through circumstances beyond her control: When the tumah occurred without the woman being aware of what she was doing, or when it was entirely against her will and there is no negligence here - the measure remains at the smaller amount, one forty-eighth, since the challah is going to be burned in any case because of its tumah.
She rendered it tamei deliberately and knowingly: The measure remains at one twenty-fourth, even though this challah too is burned just like the previous one.
The reason for the distinction: someone who was obligated in preparing this dough and deliberately allowed it to become tamei should not benefit from his act and his sin. For this reason the larger measure, one twenty-fourth, is left in place for him, and we do not say that it drops to one forty-eighth - so that the sinner should not come out ahead.
In summary: In this mishnah we learned the measure of the challah itself: one twenty-fourth of the dough, an amount considered a worthwhile gift for the kohen; one who bakes for himself and one who bakes for his son's feast are governed by the same law, while a baker and a woman who bake to sell in the market separate one forty-eighth. We also learned the law of dough that became tamei - unintentionally or beyond one's control, one forty-eighth, and deliberately, one twenty-fourth, so that the sinner should not benefit from his sin.