Length is important because a longer knife turns more slowly. This is good. It means that the knife will travel further horizontally for every turn or fraction of a turn it makes. In practical terms, it means that for any given throw, the knife will stick in the target over a wider throwing distance. That is, the difference between being too close and being too far grows larger as the knife gets longer.
Weight is important for control and momentum.
As the knife gets longer, it is affected more by air pressure. A knife that is too light must be thrown harder to insure its stable flight to the target. This can put an unnecessary strain on the throwing arm, much like trying to throw a small pebble as hard as you can. A heavier knife has more momentum even when thrown a bit more slowly. Weight, because of the momentum effect, also makes a great deal of difference in a hunting situation.