Once the Tzitzit have been tied, is there any requirement about their length?
theres a difference if you're tying the tsitsit now or if they were previously tied and now they ripped. if you are tying them now, they should be 4 finger-widths for the knots and 8 for the strings. if they rip, you need 4 strings (not half strings) that are long enough to tie a single knot in. if there is one of the original 4 strings (i.e. the 4 that became 8) which are torn to the extent that you cant tie a knot (on either side, since every string is doubled) its no longer usable. so, practically, if there are 2 of the 8 half-strings that are torn to the extent that you cannot tie a knot in them, we are concerned that they come from the same string, especially if they are on opposite sides of the last double-knot. since today, when most people tie tzitzit, they are careful to keep the same strings on different sides of the knots, (meaning you wont find the original string having to "heads" on one side of the knot), if the ripped strings are on the same side of the knot its fine and we assume they came from different original strings. this is the ruling of the shulchan aruch hope thats not to confusing.
"The length of the 8 strings [must be] not less than 4 measurements (fingerbreadths, I believe), and some say 12 [measurements], and this is how we practice; and more than that they have no [required] measurement (ie., no maximum length).
When doing my own tzizit I was told that the "tassel" must be twice as long as the "knots" So the more knots you do, the longer the tzizit need to be. But there is no maximum.
The "tassle" must not be knotted, otherwise you start counting the required length from that knot. "twisted" is not knotted.
With regards to the length, the Gemara in Tractate Menahoth discusses whether it is long enough to be able to tie it in a knot, but leave it up as a Teiku (undetermined) what the halakhah is - is it regarding one string (of the eight which is really four with both ends), or the entire tassel (out of the four tied to the corners of the garment).