I am sure this information ought to be, or ought to have been until recently, easily enough available in books... although those books might be in Chinese.

But all I could find on the Web were tantalizing hints.

The Wikipedia article on the typographical point noted that there was a Chinese system of type sizes labelled with numerals... and noted the modern type sizes assigned as approximations to them in Microsoft Word!

I had thought there must be a better source of information than that.

I found an issue of TUGboat in which a Chinese printer, reminiscing, gave different values for the numbered sizes - the article of interest is "Typesetting Chinese: a personal perspective".

And then, on Google Books, I found a preview of the book "Gutenberg in Shanghai: Chinese Print Captialism, 1876-1937" by Christopher A. Reed, which had, on page 54, an illustration of a short specimen of the four indigenous Chinese styles of modern movable type, in the numbered sizes.

I am now speculating that the sizes may be based on a common unit, nominally 1 3/4 points in size, but actually slightly smaller, perhaps exactly 1/42 of an inch.

Two-line Double Small Pica  Initial  42    24
Double Pica                 #1       28    16
Double Small Pica           #2       21    12
Two-Line Brevier            #3       16.5  10
Three-Line Diamond          #4       14     8
Small Pica                  #5       10.5   6
Brevier                     #6        8.25  5
Ruby                        #7        5.25  3
                            #8        5

The second-last column is the presumed size in slightly shrunken points, the last column the size in the basic unit.

If anyone knows the real facts here and can reveal them to me, so that I can replace my guess with the truth, I would be very happy.