Menachos chapter 6, mishnah 6. This mishnah deals with the quantity of raw material - the amount of grain - needed in order to produce the flour for the various menachos. The mishnah lists three types of menachos, and in each one the ratio between the raw grain and the flour it yields is different.
Two factors determine the ratio:
Wheat or barley: Barley has more chaff, and is therefore less efficient - a greater volume of harvested barley is required in order to produce flour from it.
New grain or old: The new crop, whether barley or wheat, contains more moisture, and it too is therefore less efficient and requires a larger quantity. Old grain, by contrast, yields more flour when ground from the very same initial volume.
The Omer offering:
The Omer is brought on the second day of Pesach, and in it both disadvantages come together: first, it comes from barley and not from wheat, and barley is less efficient; second, it is the brand new barley crop, which is also less efficient. Therefore the mishnah teaches: "Ha'omer hayah ba isaron mishalosh se'in" - one isaron of flour is produced from three se'ah of grain, which is a very large quantity.
Three se'ah equal one eifah. As you will recall from the conversion mnemonics: alef-samech marks the step from the eifah to the se'ah, and the gimmel of "gadu" indicates a factor of three - three se'ah in one eifah. And for a concrete picture: a se'ah is like a case of six two-liter bottles, and an eifah is three such cases.
It emerges that one must begin with a full eifah of barley kernels. "Isaron" means a tenth of an eifah, that is, for every ten units of raw kernels one unit of flour is obtained. Since an isaron of flour is required, one must start with ten esronos - a full eifah - in order to produce the flour needed for the Omer offering. The ratio: ten to one.
The Two Loaves:
The Two Loaves are the two wheat loaves waved on Shavuos. They too come from the new crop, which is one disadvantage, but they are wheat and not barley. Each of the two loaves comes from an isaron of flour, two esronos in all. And the mishnah teaches: "Shtei halechem shnei esronos mishalosh se'in" - the two esronos are produced from three se'ah, which are one eifah. It emerges that one eifah yields two esronos, and the ratio is not ten to one but five to one, which is more efficient.
The Lechem Hapanim:
The Lechem Hapanim is the weekly bread offering. It comes from wheat, and it is also not from the new grain but from the old, so that both advantages are present in it. These loaves are very large: twelve loaves, each one requiring two esronos of flour, as is derived from the verses, twenty-four esronos in all. And the mishnah teaches: "Lechem hapanim esrim ve'arba'ah esronos me'esrim ve'arba se'in" - one se'ah for each isaron. And since a se'ah is a third of an eifah, the ratio comes out to three and a third to one.
In summary: These are the opening quantities required lechatchilah - a full eifah for the Omer offering, a full eifah for the Two Loaves, and twenty-four se'ah (a se'ah for each isaron) for the Lechem Hapanim. But bedieved, as long as the correct amount of flour was ultimately obtained, the minchah is valid, whether one began with a large amount of raw material or with a small one.