This is measured from your closed fist to your elbow this will give you the correct size for you, the ends are rounded smoothly and the diameter is the circumference from your middle finger to your thumb, it looks like a large rolling pin.
This is about 1/3 metre long and has a ball in the middle with the two arms tapering to the end where there is a circle, shaped into a dome that will cover the centre of the hand.
Within the theory of Taiji Stick and Ruler, the stick pertains to Yang and the ruler pertains to Yin. When practicing, both hands must stay in contact with the syick or ruler and the fact that you have something to touch helps with visualising in moving the energy. You can make a connection with the eyes and movements and the Dan Tian. For those of us that practice the Hunyuan Chen Shi Xin Yi Taiji this is normal as it is the way we practice the form also, guided externally connected internally.
http://www.charlestauber.com/sticksandrulers/index.html for image