The only real way to get an exact size is to use a longer than necessary arrow in the bow you intend to use, draw the arrow to your anchor point, then get someone to mark the arrow at the point where it intersects with the bow's back. This would give you the minimum length shaft from the nock to the shoulder of the point.
You only need to be this accurate if you are going to be target shooting and are looking for the perfect arrow for this otherwise do as pappy says only instead of measuring the length directly I have found it easyier to use a long shaft/stick and place one end in the middle of your chest and hold the other by your fingertips with your two arms streatched out in front of you. The get someone to mark the shaft at the end of your fingers.
I must add that unless you are after the best target arrows you can make then there is no "proper arrow length", there are plenty of people who deliberately shoot longer arrows than are rfequired by their draw, they do this because they prefer the way the arrows fly or they need longer length to get proper dynamic spine etc, etc.
Craig
