It is not a question of how long the clips are in total but whether there is enough 'unused' video beyond the existing transition to allow the transition's duration to be increased without having to shorten the total duration of the two clips with the extended transition.
It's s bit difficult to explain. When you first add a transition and there is not enough unused video for the required overlap it warns you that it is going to shorten the clip in order to provide this overlap. When afterwards you tried to increase the transition duration in past versions it did not let you unless you trimmed the clips back first yourself.
I have just tried increasing a transition duration from 1 to 2 seconds in one of my projects now I'm using iMovie 10.1.2 and I got a warning that the clips would have to be (further) shortened - which is understandable:
However on clicking OK the warning was dismissed but the transition remained at 1 second so this does not appear to be working.
I then shortened the two clips either side of the transition and tried again. This time no warning appeared and the transition duration went to 2 seconds.
I hope this is clear. If you want say a 2 second transition the clips on both sides have to have at least 2 seconds of 'unused' duration to provide the overlap that the transition requires without decreasing the overall duration of the project. There is no such problem with stills since they can be extended to any duration.
Geoff.