The size of a PDF is generally defined by the CropBox or MediaBox setting for each page. This is the number of pixels. This provides a set of 4 number (x,y,width,height) to define how big the page is. A common value is 0 0 595 842 for an A4 page.
However, most people are interested in actual units and that is what Adobe Reader or Acrobat displays. Here is the output from a file with CropBox[0 0 585 832] and MediaBox[0 0 585 832]
So where does this number of centimetres come from?
Standard dpi is 72 dots per inch so we can convert the CropBox and MediaBox to inches by dividing these numbers by 72. This gives us 8.125 inches by 11.556 inches.
There are 2.54 centimetres in an inch so multiplying by this we get 20.635 cm by 29.35cm
So that is how Adobe creates the size from the raw PDF Crop or Media box sizes.
If you are interested in using JPedal to generate the size values of each page, there is a simple code example in this post.