How to Stop text resizing in PowerPoint for all presentation

To stop text resizing for all PowerPoint presentations (current and new presentations):

1. Click the File tab in the Ribbon.

2. Click Options. A dialog box appears.

3. Select Proofing in the categories on the left.

4. Click AutoCorrect Options. A dialog box appears.

5. Select the AutoFormat As You Type tab.

6. Deselect (uncheck) the AutoFit title text to placeholder and AutoFit body text to placeholder options.

7. Click OK twice.

Ref: https://www.avantixlearning.ca/microsoft-powerpoint/training-how-to-turn-off-automatic-text-resizing-in-powerpoint/