The Gemara in Menahot establishes the minimum measure for the length of Sisit as four "Gudalim"-thumb-widths. While this may seem like a simple standard, there is a major controversy how to understand it. The Gemara also establishes that the ratio between the "G'dil"- the knotted portion of the tassel and the "P'til"- the loose strings- should be one to two. That is, one-third of the total length should be G'dil and two-thirds should be P'til.
Rashi understands the Gemara's measure of four thumb-widths as referring to the loose strings of the P'til. Since the P'til is two thirds of the entire tassel, the total requisite length would be six thumb-breadths. Rambam understands that the measure of four thumb-widths refers to the total length of the Sisit, which would be divided one-third G'dil and two-thirds P'til. Rabbenu Tam understands the measure as referring to the G'dil, which is one third of the entire tassel. Therefore, the minimum length of the entire tassel would be twelve thumb-breadths. The Shulhan Aruch states that the custom is in accordance with Rabbenu Tam.
The length is not measured from the hole, but from the bottom edge of the Tallit. In contemporary units, twelve thumb-breadths equals 24 centimeters, which is 9.6 inches. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) in Parashat Noach says that if the Sisit were at the minimal length, but then shrunk, it is permitted B'dieved-after the fact. There is no maximum length for the Sisit.
The Hazon Ish (Rav Abraham Yeshaya Karelitz, 1878-1953) held a stricter position regarding the measurement of the Gudal, which came out to 11.4 inches. Hacham Ovadia ruled that one does not have to take that opinion into account, since first of all there are more lenient opinions, like the Rambam, who maintains that the Sisit only require four Gudalim, and even if twelve Gudalim are required, there is still the opinion of the Ben Ish Hai that B'dieved, after the fact, it is valid.
If one tied strings and the P'til was longer than the proper ratio of two thirds, it is permitted to cut the strings, and this does not constitute a prohibition of "Ta'aseh V'lo Min Ha'asuy," as the Sisit were already valid, B'dieved, after the fact. However, the Poskim state that one should not cut the strings with a metal instrument. Rather, one should use his teeth or ceramic scissors.
The minimum length of the Sisit strings is 9.6 inches. The G'dil should be one-third the total length and the P'til should be two-thirds. If the Sisit need to be cut to achieve the proper ratio, metal instruments should not be used.