I'm Getting Started, Slideshow Project

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@HikingMike

I wasn’t able to find any way to adjust the width/height proportions on the viewer. I did see someone else in the forum say that the player maintains the aspect ratio of the source video.

That is the case when Settings > Video Mode = Automatic (and the first thing added to Playlist or Timeline is a video). If you want control - highly recommended - change Settings > Video Mode to something that you want before starting a project.