WRT the project length, the setting in the prefs is for the default length of new projects. It has no effect on the current project. Unfortunately it also has no effect on new projects until you quit and relaunch Motion.
When you create a new project, in the project browser, it shows the length of the project that you're going to create. You can change it there.
If you already created the project and it's too long (or short), you can change the length of the project in a couple of places.
Select the Project in the layers list and show its properties in the inspector (or just hit cmd+j to select and show the project properties).
- Set the Duration to what you need.
Click the clock widget in the center of the tool bar. This switches between showing the current time to the project duration. When it's showing duration, double click in the frame/timecode and type in the new project duration. You can also switch between current and duration via the little downward arrow next to the frames/timecode display.