If your plumbing fittings fail to adjust their size properly when you place them or when you trim or otherwise connect pipe it is time to get suspicious about the path for your Lookup Tables. Revit installs fitting lookup files that match the name of each fitting supplied in a folder called "Lookup Tables" in your content folder location. Each file is a comma delimited text file which means the information is separated by commas and the file extension is .csv.
For stand alone installations the default location is:
C:\Documents and Settings\All Users\Application Data\Autodesk\RME 2008\LookupTables
Network installations could be anywhere from the "D-Z drive" unfortunately.
Regardless this location is stored in your Revit.ini file located here (default installation location):
C:\Program Files\Revit MEP 2008\Program
If the actual location and the one stored in Revit.ini are different then your fittings will not resize graphically until they match. You'll need to close and restart Revit to get it to re-read the .ini file.
Last, there is no place within the Revit user interface to check or alter this location which I think ought to be changed and included within the Mechanical Settings dialog in the same way the Pipe Sizes.xml location is presented.