By default, the OBS preview window will be 1920x1080. The new MacBook Airs have a screen resolution of 2560x1600. This explains why the window is only showing the top corner of your screen. Your screen has more pixels than OBS is being told to capture. There are 2 ways to fix this.
1.) Resize the preview/source window resolution to the resolution of your screen (2560x1600). This will allow you to capture the entire screen, however will not be the "standard 16:9 aspect ratio." Here is how to do this:
A.) After setting up your display capture, go to your OBS settings.
B.) From there, go to "Video:"
C.) Change the BASE (Canvas) Resolution to 2560x1600. Also, change the Output (Scaled) Resolution to 2560x1600.
You may end up having to adjust the sizing depending on your screen resolution. You can see the steps on how to do that in the option (#2) that follows.
2.) Resize your display capture window to be 1080p. This will allow you to still capture in 16:9, however, the MacBook screens are NOT the standard 16:9 so there WILL still be some portions of the screen that are cut off (or have black bars). Here is how you do that.
A.) Open OBS and set up a display capture so that the screen still appears in the top left corner.
B.) Select the red dot in the corner of the screen and drag it to shrink the window.
C.) Adjust the fitting accordingly and to your desire for your recording/stream.
I hope this helps! Let me know if you have any questions.