A presentation made for a widescreen display may need a squarer canvas for another venue. PowerPoint changes slide size for the whole presentation. It does not give each slide a separate canvas size. Save a copy before changing the format so you can return to the original layout.
Choose the destination first
Ask which aspect ratio the destination expects. In current desktop PowerPoint for Windows, open Design → Slide Size and choose Standard (4:3), Widescreen (16:9) or Custom Slide Size. These options describe the slide’s shape, not the resolution or sharpness of every image inside it. Menu labels and scaling dialogs differ on Mac; Microsoft’s guide has separate platform sections.
For example, a title bar that reaches both sides of a 16:9 slide may not fit the same way in 4:3. Changing size does not redesign the composition for you.
Understand the two choices
When Windows PowerPoint asks how to scale, Maximize enlarges the content to use the available space and can put some elements outside the slide. Ensure Fit reduces it so all content fits. The second choice is a useful starting point when preserving everything matters, but it can leave small text or spare space that needs manual work.
Check a diagram, a wide table and a full-slide photograph first. Move or resize elements after the conversion. If a picture needs cropping, preserve its proportions rather than stretching people or logos to fill the gap.
Inspect the saved result
Review every slide in Slide Show view, especially titles, footnotes, page numbers and objects near the edges. If you will deliver a PDF, export the changed copy and open that actual PDF too. Verify both the first and last slides and the widest table, not just the thumbnail overview.
Keep the original and converted presentation under different names. If the venue only needs a different print layout, check the print or handout options before changing the slide canvas at all. Slide size and printer margins are separate settings.