This is a repeat of a previous post with some small additions.
In this coming weeks Parsha, Kedoshim, the Torah has the din of פאות הראש. The main sugya relating to these dinim is in מכות כ. I would like to discuss some of the halachos:
I. Where do the פאות הראש end?
Rashi on the mishna in מכות כ comments that the head is divided into 2 parts, the hair and the face. The dividing line is the bone on the side of the ear. In other words, פאות הראש are up until the middle of the ear. Rashi says this on the gemara as well and in Shavuos (6a) also. The Tur paskens this way. However the Beis Yosef found another Rashi on the mishna, where Rashi writes that פאות הזקן start below the ear and anything above that is פאות הראש. The Shulchan Aruch in סימן קפ"א is machmir like this opinion.
However, many acharonim disagree and point out that the the Shulchan Aruch is like none of the rishonim. The ארץ הצבי brings an interesting proof. He points out that people whose beard is a different color then their hair, the changeover occurs in the middle of the ear and therefore we see clearly that the middle of the ear is the dividing line. This seems to be the accepted minhag.
II. How much hair needs to be cut off?
There is a machlokes the Rambam and the Semag from one end of the spectrum to the other. The semag based on a Tosefta in מכות פרק ד says that if you cut off even 2 hairs in the area of פאות הראש you are chayav. The Rambam seems to hold as follows. Min Hatorah, you only violate the issur if you cut off all the hair, if you leave even 2 hairs you are not chayav. M'drabbanan you need to leave either 4 or 40 hairs. Rashi sounds like he agrees with the Rambam
III. How can you cut the hair?
Again a machlokes, the Rosh and the Rambam. The Rambam holds that it is just like פאות הזקן, you are only chayav with a razor. The Rosh holds that you are chayav even with מספרים כעין תער.
IV. What is the shiur for each hair?
According to the Rosh that you are chayav for מספרים כעין תער how long does each hair have to be? The poskim point out that the shiur of hair in general is to bend back the hair on itself, about a half a centimeter. In other words, ע"פ דין, there is absolutely no source for long payists. Each hair has to a 1/2 centimeter long, that is it. There is no chiyuv whatsoever to grow each hair long.
Some want to say that growing each hair long is a hidur mitzva, however this is very difficult. פאות הראש is a לאו, by a לאו it is difficult to say there is such a thing as hidur mitzva. Either you violated the din or you didn't. If you didn't violate the din what exactly is a hidur going to do. Others explain that it is done as a סימן, especially for little kids.