Instructions
Obtain a few darts in every length and practice with them. There are six standard shaft lengths: Long, Medium, In-between or Mid, Short, X-short and Micro. The exact measurements vary between manufacturers, but the six sizes generally range between the extremes of 1/2 and 2-1/2 inches.
Throw a few darts at the board. Observe how they fly through the air. If your darts are wobbling as they fly, you likely need a shorter shaft.
Note the angle at which the darts strike the board. Ideally they should hit at a 90 degree angle, or angled slightly upward. If they are pointed too far downward, use a longer shaft. If your darts strike at too low an angle, pointing upward, use a shorter shaft.