John,
There is a description of the steam pipes in the report of the Board of Trade inquiry. You can find it on line at the Titanic Inquiry Project. The 1910 Olympic/Titanic article in the British journal "Engineering" has more information and very detailed plans of both engine rooms and boiler room #5 & #6, including a great deal of piping. I don't know of any drawings of the steam pipes forward of BR#5. There used to be a nice site with some of the "Engineering" article online, but it is no longer at the old URL. Many large libraries have "Engineering" in their collections, look for "Engineering (London, England)".
The Olympic/Titanic special number of "The Shipbuilder" also discusses the piping. Amereon House reprinted it, but I don't know if the book is still in print. You might try inter-library loan.
This site has some low resolution scans of some of the plans from "Engineering" and elsewhere. You can at least get an idea what's available.
http://www.abratis.de/sources/pictures/blue.html
The diameter of the main stop valves was 21-1/2". I'm not sure that the exact diameters of the steam mains are given anywhere, but they reduced in diameter as the mains went forward.
It's not really clear how the steam mains could be connected direct to the condensers. The BOT report mentions a "silent blow-off", but none of the drawings that were published shows any valving that would fill the bill. There was a "turbine start valve" that could allow HP steam to be routed to the condensers, but it was only on one side and fairly small in diameter.
Regards,
Cal