Reddit reviews Special Taoist Taiji Stick and Ruler Qigong

There are principles and techniques to apply. There is the yang Taiji bang vs the yin taiji ruler, and the postures for static zhan zhuang differ for each. The dynamic qi building exercises differ even more. You might want to do some research to learn from a master who actually teaches these techniques in depth.

Specific to your question: the Taiji bang should be cylindrical, rounded at the ends, the length of your wrist to your elbow, and the diameter of your wrist. It can be held with palms flat against the ends for zhan zhuang, so a rolling pin might not work. It can also be held around the ends as you would a staff or baton, and the wrist like diameter is intended to strengthen your grip for qinna or grappling as well.

This is an image of both the bang and ruler. The shorter stick is the ruler.
https://images.app.goo.gl/geUZk5LQstrhfYQXA

Look for instructional material like this: https://www.amazon.com/Special-Taoist-Taiji-Stick-Qigong/dp/B0019TPE34

You should be able to find Master Wang Feng Ming's website if you want to purchase a copy directly.

Special note: there is a very cringe worthy video I've seen of someone teaching exercises he created himself while holding a home depot wooden ruler, probably because he's heard the term Taiji ruler. Be selective and discerning of the information you find on the net, including mine.