If you want to globally crop your presentation, you can do so with the Crop Canvas feature.
You would use this feature if you didn’t want to show the whole screen recording. For example, if your screencast is going to show one area of your desktop, you could use the Crop Canvas to accomplish this task.
To crop the canvas, click on the 
button in the main
window. The preview area will then look like the following:
To change the cropping, move the mouse to the edges of the
preview, click down and move toward the center. When you are done,
press the Apply
button, and the canvas size will change.
Snap to Front Window
If this option is checked, then guidelines will be presented for
the foreground window. This allows you to very quickly globally crop
the presentation to the front-most window.
Reset
Reset to original canvas size, removing any cropping.
Cancel
Cancels the current crop editing session, restoring the previous
cropping values.
Apply
Applies the current global crop values to the presentation and
ends the crop editing session.
