There seems to be a minor issue setting the transition length in MEP 2014.
Place two short clips on the storyboard - the second one less than 20 seconds in duration.
Click on the cut icon between them and select Crossfade.
Now click again and select set transition length, then set a duration great than the duration of the second clip.
The transition is set to the maximum possible - a frame less then the duration of clip 2. All good so far.
Now try to adjust the duration again - the menu option is greyed out!
In MEP 2013 it's possible to still adjust the transition duration.
A sample project here: https://app.box.com/s/m9j8huubpm1tmuvtynlo