One of the oldest depictions of a Zheng-like instrument, created from remnants found in a tomb in  Changqiao,Wu Xian, Jiangsu Province. The tomb was dated to the fifth century BCE. It had twelve strings, was made of wood, and was coated in lacquer. It's 132.8 cm or ~52 inches long.

Source: Smithsonian Institute, Washington D.C. The Smithsonian Institute further attributes the image to Huang Xiangpeng 1996, Zhongguo yinyue wenwu daxi, volume on Jiangsu/Shanghai: 248-51, Zhengzhou: Daxiang Press.

And here is an image of one such early zheng! This was one of the ones pulled from a tomb, but the specifics are unclear. You can see it in context at yueqixuexi.com, （乐器学习, "Musical Instruments", Chinese)

Source: From yueqixuexi.com, Used in accordance with fair use exception of China and US copyright law.