Shabbos, Chapter 13, Mishnah 4. This mishnah deals with the shiur - the amount of work required in order to be liable, when dealing with threads and their production.
The measure for the labors that prepare the thread:
The words of the mishnah: "HaMelaben v'hamenapetz v'hatzovea v'hatoveh - shiuro k'melo rochav hasit kaful" - one who whitens, one who combs, one who dyes and one who spins - the measure for each is a full double sit. Four labors are listed here:
"HaMelaben" - whitening the material used to make the thread.
"HaMenapetz" - combing out the wool, as part of the process of preparing it.
"V'hatzovea" - dyeing the material, dyeing the wool.
"V'hatoveh" - spinning the wool and turning it into thread.
In all four of these labors - whitening, combing, dyeing and spinning - the measure is one and the same: a full double sit.
What is a "full sit"?
The sit is the distance created when one spreads the index finger away from the middle finger, and that distance is a full sit. A "full double sit" is twice that measure. It is worth noting that the distance between the thumb and the index finger is exactly double the distance between the index finger and the middle finger, so that the two expressions refer to the very same measure.
"V'ha'oreg shnei chutin":
The measure for the labor of weaving is somewhat different: "V'ha'oreg shnei chutin - shiuro k'melo hasit" - one who weaves two threads, his measure is a full sit. The amount that the weft thread must be passed through the warp threads is only the length of a full sit - the space between the index finger and the middle finger, half of the previous measure. There is no need to pass the thread all the way to the end, and even passing this small amount, which has actually been woven, is enough to be liable for the labor of weaving on Shabbos.
In summary: In this mishnah we learned the measures for the labors involving thread: one who whitens, one who combs, one who dyes and one who spins - their measure is a full double sit, while one who weaves two threads - his measure is only a full sit, half of the measure in the preceding labors.