We only have NavisWorks Manage in our office so this answer is coming from that perspective. You should be able to select the object in question using the Selection tool and then open your Properties dialog box. The size of the pipe should be listed in the Properties dialog box once the item is selected. I hope this helps.
If this property is something you want quick access to all the time, you can customize Smart Tags to include it using the Global Options interface. Then with Smart Tags turned on, you'll get a tooltip with the information whenever you move the mouse over an object.
Tim Wiegand Software Architect Navisworks Autodesk, Inc.