23-25 inches, select the 23-inch reach stick. If your "full arm" reach is 25 inches or greater, select the 25-inch reach stick.  All three tree scale sticks can be downloaded from the Illinois 4-H Forestry Judging Contest web site address below:

http://forestryjudging.nres.uiuc.edu/page73.html

Tree diameters are measured at 4 1/2 feet above the ground, a point referred to as diameter breast height (DBH).  Determine where the 4 1/2 foot mark is on your chest/neck area.  You can use the yardstick to determine this measurement.

Assuming you have selected the appropriate tree scale stick for your reach, stand facing the tree's trunk and position your body so it is in the center of the trunk. Hold the tree scale stick horizontally against the tree at 4 1/2 feet above the ground line and push yourself back away from the tree to the arm's length reach that your scale stick is calibrated for; 20-, 23- or 25-inches (see photo 2 and 2a).  It is important that distance between your eye and the scale stick be 20, 23 or 25 inches depending on the stick's calibration.  If this distance is less or greater than what the stick is calibrated for, the diameter measurement you determine will be wrong.

Photo 2 and 2a.  To Measure the Tree's Diameter, Position Yourself in the Center of the Trunk.  Hold the Scale Stick Against the Tree at 4 1/2 Feet Above the Ground Line and Push Yourself Back Away From the Tree the Required Distance That Your Scale Stick is Calibrated For; 20 inches, 23 inches or 25 inches.  Click Photos to Enlarge - 2 (32 KB), 2a (31 KB).

Make sure you hold the scale stick firmly against the bark of the tree. Now shift the stick right or left until the zero end of the stick is flush with the left edge of the tree trunk. Without moving your head (move only your eyes, not your head), look over to the right edge of the tree's trunk and read