My ties are about 1" - 1.25" in length, but I don't use yarn. I use crochet thread or embroidery floss doubled, so there are 4 threads in the knot. I have seen shorter, and I have seen longer. I think it is a matter of personal preference, as long as the knot is secure. I don't know how to exactly describe the knot that I use, but the needle does the work, and the tying takes place before the cutting, resulting in almost no waste -- just what is left on the needle at the end when it isn't long enough to do any more.
Dayle