A deck prepared for a different screen shape may need more than a display setting change. Changing the presentation’s aspect ratio changes the canvas on which every slide is laid out. Preserve the original before adapting it for a new venue or template.

Confirm the required shape first. In desktop PowerPoint for Windows, Design, Slide Size offers Standard 4:3, Widescreen 16:9 and custom dimensions. The size applies to the presentation as a whole; it is not a per-slide setting.

Choose a fit strategy, then inspect its result

When PowerPoint asks how to scale existing content, Maximize can enlarge content so that some no longer fits on the slide. Ensure Fit scales content to fit the new canvas, which can leave it smaller than you want.

Use the option that gives the better starting point for the actual deck, then review the slides individually. Neither option replaces layout judgement. Undo and compare the alternative in a separate copy if the initial result is unsuitable.

Start with the slides most likely to reveal problems: full-width tables, diagrams near an edge, portrait photographs and titles that already wrap. Check captions and footer text as well as the central content.

Judge the adapted deck at presentation size

Run the slideshow on a display with the intended shape. Check whether an audience can still read the smallest essential label. Reposition or simplify crowded material deliberately instead of shrinking everything again to make it technically fit.

Compare the adapted deck with the original for missing content and changed emphasis. A chart that dominates the old layout may become visually minor after automatic scaling, even when nothing is clipped.

Save the approved version with its intended format in the filename or handover note. Retain the original if it is still needed for another venue. The completion check is a readable, complete presentation on the target canvas, rather than merely a widescreen value in the Slide Size dialog.

References: Microsoft: changing slide size.