When drawing pipe and duct you can press the Space Bar to acquire the size of the duct and pipe. Further when used on Pipe Fittings it can be used to orient a new section of pipe to the workplane of a fitting, for example the double wye or reducing wye.

Place your first point on the connector snap symbol and move your cursor away from the first point in the direction you want to the duct/pipe to extend and press the Space Bar. You should see the size of duct change and in some cases Revit will change the background color of the screen to blue when working on Pipe Fittings.