Personally, I reckon it would have been used in the Dining Saloon - as David suggested - and it was closer to six feet than twelve. The individual leaf in the programme was approximately 2' by 6', and thus I suppose the addition of three leaves would extend the table's length to 12'.
Are there any other examples of special tables designed and designated for the Captain's table - is there precedent? It is my understanding from several conversations that all the tables were have been identical on these liners of the day. It is a shame both the owners and Roadshow "experts" (recall the "menu"...) gave a description disregarding reality in any event.
This past Sat. evening the Roadshow featured a walnut bookcase, circa 1750. The woman with the piece had told the appraiser that the cabinet maker died on or about the time of finishing the bookcase. Therefore, the woman's Mother cancelled her booked passage from Europe to the U.S. via TITANIC. The passage was then moved up to Aug. 1912.