Hi, I had a similar problem. Udemy recommends 16:9 aspect ratio for videos. So, if you are having MacOS version Sierra - in system preferences, change the resolution to 16:9 while recording your screen. (I can give you exact steps to do this, if you don't get it right) You will see black bars at the top and bottom. Don't worry. This will not appear in the video. Your video will come out with an aspect ratio of 16:9 only. In case you are using MacOS Mojave version, it does not allow you to change the aspect ratio, you will have to use third party softwares (like ScreenRes) to do that. Hope this helps
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