If typed contents reach the boundary of a text field for the title of flowing text, PowerPoint automatically reduces the font size. This is often helpful, but can be very annoying Even when the font size is basically a format, the function for automatic adjustment of this setting is located somewhere else.

Open the "Tools I AutoCorrect Options" command just next to the "Format" menu and activate will find both options "Auto Fit title text to placeholder" and "Auto Fit body text to placeholder" in the "Apply as you type" section. These options help you define whether and for which text types Power Point should automatically adjust the font size.