@livmarx I am not sure which video editor you are using when you are editing a video, but if it is the Screencast-O-Matic/ScreenPal tool you should refer to the tutorial videos for using the tool as it may help. This one on editing the timeline might be helpful. Additionally, with Screencast-O-Matic/ScreenPal you can preset the video size when recording in the Capture Settings.
If you are using Studio Capture to record, you may want to consider using Screencast-O-Matic/ScreenPal instead as it will allow you to specify the screen size.
If you are using a different video editor, you may want to contact their support.