He [Shi Hunagdi] himself wore black, while his troops in black armour issued from black-flagged fortifications beneath black-emblazoned standards. … The number six posed no problem. The interlocking tally-sticks that signified an imperial commission were ordered to be six 'inches' wide; and the length of a 'pace' was calculated as exactly six 'feet' (it was a double pace, or two strides). Six 'feet' was also the prescribed width of official carriages. [emphasis author's, not mine]
--China: A History, John Keay, pg. 91 (third paragraph), 2008-2009 print edition.
Now, it seems that he has defined the "feet" in the literal sense of the word. But counting in double strides, it would then become "three strides" not "six feet". And he hasn't clarified what he means by "inches".
Around that time, according to wikipedia, the Chinese measurements were 尺, 步, and 里 --none of whose six counts come close to acceptable widths of tally-sticks or carriages.
Because the author specifies no sources for this speculation I reach out to Chinese history experts on this SE to get their opinion on the correctness of his claim and perhaps even clarifications on what he may have meant by "inches" and "feet".
P.S.:- To those who may wonder why "six" is important to this question --the colour black, the season winter, and the number six are associated with the Chinese element "Water". The Qin's associated element was "Water" as they had displaced the Eastern Zhou (周朝) whose element was fire.
Clarification: The author's writing style is such that he encloses modern equivalents of ancient Chinese terms in single quotes and omits the mention of the ancient Chinese term (presumably to maintain an easy reading and avoid unnecessary deviations from the main analysis). Thus --'counties', 'courts', and 'feet'.