You can use a transition effect with any visual clip regardless of the layout
you have defined, or whether another clip precedes or follows the clip that uses
the effect. When a clip starts, the area it covers is treated as its background,
whether that area is a region color or a clip in another region, or a clip in the
same region. A transition effect simply introduces the clip over, or removes the
clip from, its background. So the clips do not have to be the same size,
when you use transitions with a sequence of clips,.