Sukkah, Chapter 3, Mishnah 7. The Mishnah deals with the required size of the esrog: what is the acceptable measurement, both for the smallest esrog that can be valid and for the largest.
The size of a small esrog:
"Rabbi Meir omer: ka'egoz" - Rabbi Meir holds that an esrog may be as small as a nut, and apparently the reference is to what we call a hazelnut.
"Rabbi Yehudah omer: k'beitzah" - it may be as small as an egg, but no smaller than that.
The size of a large esrog:
"Kedei sheyochaz shnayim b'yado achas" - this is the opinion of Rabbi Yehudah: as long as a person can hold two esrogim in one hand, the esrog is valid; but if it is impossible to hold two in one hand, it is too large.
"Rabbi Yose omer: afilu echad bishtei yadav" - on the contrary, an esrog may be so large that two hands are needed to hold a single one. In other words, even a very, very large esrog is valid, and this is how the halachah is decided in practice.
In summary: in this Mishnah we learned the measurements of the esrog. Regarding a small esrog, Rabbi Meir (the size of a nut) and Rabbi Yehudah (the size of an egg) disagree, and regarding a large one, Rabbi Yehudah (two in one hand) and Rabbi Yose (even one in two hands) disagree, and the halachah follows Rabbi Yose.