I suggest you go to a reputable proshop, and get your draw length checked.
Just for a quick check stand with your left arm outstretched in a fist configuration, and hold it against a wall. Now measure from the wall to the corner of your mouth. This should give you a rough estimate of your draw length. Be sure to hold your head upright and relax. Hope this helps.
The yardstick method is to put it under your chin and put both arms straight out in front of you palms together with the yardstick between them. The measurement from your neck to the tips of your fingers is a good starting point.