My 15" Macbook Pro Retina screen is 2880x1800 res (16:10 aspect ratio), but how do I select a recording area that is exactly 16:9 aspect ratio with say a resolution of 1920x1080 for recording purposes - so that I can finally export/produce to 1280x720 res. (16:9 aspect ratio)?
The easiest way is when the recorder UI is showing, in the left hand panel where you choose to record the screen, in the drop-down menu (where your screen is listed), choose one the the preset regions. Your monitor will then dim out and show the the region that we will be recording. You can click and drag inside that region to move it around your screen to encompass the are you wish to record. Once it is place where you want it, hit OK. The recorder UI show up again and you can choose record to start recording. While recording, there will be a line around the recording region so you can visually tell what part of the screen is being recorded. The nice thing is, if you normally want to record the same area of your screen, we remember your last selected region, so when you do subsequent recordings, the last used region will automatically be selected. You can always use the drop down to pick a different region and/or monitor any time you want to change it. You last 3 custom region settings are stored and loaded in the the drop-down menu as well, so you can easily switch between them as needed.