On the upper right of the garage band screen there are two important small controls. One is a little slider in a frame. It is used to squash or expand the project tracks horizontally. So, you can zoom out to see the tracks from beginning to end in their entirety in the project window - or you can zoom in if you have to work on a specific more detailed part.
The other is a little triangle thingy that's often hard to click on because they put is so close to the other one. It's a little bit above and to the right of the aforementioned control. Sometimes you have to zoom out to see the entire project to see it.
This is the stop point. When exporting a project this is where the time bar will stop. Just click, hold and drag it. Once it is set at the end of the tracks, the tracks will always tend to fill the project window completely.
Happy recording. It's a great blessing to be able to do so much on this great DAW (digital analog workstation) called Garageband.