NewPowerPoint
PowerPoint: Set slide size in a modern task pane
PowerPoint is introducing a modern task pane for setting slide sizes with real-time preview, including new social media presets. This new feature streamlines the slide size workflow without disrupting editing.
Microsoft's description
Change slide size in PowerPoint from a modern task pane that keeps you in the flow of editing with real-time slide changes. Choose common slide sizes including new social media presets, or customize dimensions with a cleaner, more approachable experience.