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.