Question: How long and thick must my sideburns be?
Answer: The Shulchan Aruch (YD 181:9) writes that the area of peyos is across from the hair of the forehead, until below the ear where the lower jaw extends. The acharonim debate these exact parameters.
R’ Yaakov Kaminetzky (quoted by R’ Yisroel Belsky, Shulchan Halevi p122) explains that the upper limit of the peyos begins at the highest point of the hairline as it arches over the ear and extends in a slightly curved line across to where the hairline of the forehead turns sharply downwards towards the sideburns. All the hair from the imaginary line that connect these two points and below comprises the peyos (See Leshichno Sidrashu p269).
While many let their sideburns grow until the bottom of their ear, R’ Aryeh Lebowitz (YUTorah.org, Halachos of Peyos Harosh) writes that many poskim understand the Shulchan Aruch to be referring to below the ear canal (See Az Nidberu 3:45:7). Thus one should leave them to at least the bone that can be felt in front of one’s ear (See Leshichno Sidrashu p 269; Eretz Zvi 3:5).
The Shulchan Aruch (YD 181:10) writes that one must not place scissors against one’s face and cut their peyos. There is a machlokes as to how long the hairs must be, though.
The Chafetz Chaim (Biur Halacha 251:2) holds that the slightest bit is okay (See Tosafos Nazir 40a; Perisha 181) while R' Yitzchak Yaakov Weiss (Minchas Yitzchak 4:113:5) holds that the hairs must be long enough that they can be folded over to touch its root (see Darchei Teshuvah 181:15). Contemporary poskim advise using a #3 clipper, and no less than a #2, to ensure that the hairs are at least 5 or 6mm long.For a diagram, see www.kosherhaircut.com