For all the interested. I found a workaround:
If you need a specific size of pdf printed/exported from ‘Numbers’, first of all add that size to the custom paper sizes list with ‘Pages’ or ‘Preview’.
Then find the standard size paper that comes closest to the lengths / width ratio of the added custom paper size.
From ‘Numbers’ print/export the table to the found standard paper size with the right ratio.
Open that .pdf in ‘Preview’ and export it once again to the earlier added custom paper size and your done.
After creating all my 252 pages with tables I stuck them in a single file and then opened that file with ‘Preview’ and found that the custom paper sizes are available! So I reprinted the document to the desired custom paper size and the book printing shop accepted the pdf because all the pages ware the right size.
So thanks for your comment and hopefully they add the custom paper option to ‘Numbers’ once again but till then this workaround helps me out.