A fairly common comment from someone using the Piping/Plumbing features in Revit MEP is, "Where is Cast Iron?" Fair enough! Where is it? It ain't there! Getting it there is a matter of editing the Pipe Sizes.xml file (default location: C:\Program Files\Revit MEP 2008\Program). At least that's true for past versions. The information is now embedded in Revit MEP 2009 and accessible from a dialog found under Settings menu > Mechanical Settings. This is the dialog you get:
You click on "Sizes" at the bottom left of the dialog and you get the information displayed on the right side. You can now add Material, Connection, Schedule/Type as well as Sizes. Using the check boxes offered, you can filter the list of sizes so only appropriate ones will be offered to you when you use specific types of pipe. You also use this to allow RME to use the size when calculating "sizing" for you.
Keep in mind that the Project Units of your project will affect the pipe sizes displayed on the Options Bar too. You adjust this via Settings menu > Project Units and then choose Piping for Discipline. Like this:
Similarly Wires Sizes and Duct Sizes are embedded within RME.