Re: layer length in changed composition length.

It depends on the kind of layers they are. Most layers, solids, pics, etc. can be extended with Alt ].

So "End" get the time Indicator to the end of the comp, Ctrl A, selects all layers (deselect any of the ones you don't want lengthened), then hit Alt ].

So layers like video, that have a run time less than the extension you want won't lengthen past their run time, but most other layers will work with Alt ].

If you're on a Mac it Option ].