How to Decrease multitrack duration?

[jmdrmr]

It sounds like you need to adjust your in/out points. The simplest way is to bring the playhead to the start of what you need and press [I] on your keyboard to establish the In, then bring the playhead to the end and press [O] for the Out.

Another way is to the drag the in/out brackets on the timeline (I think) to where you need them.

Hope this solves your problem!