To change the default font and font size in PowerPoint, follow these steps:

1. Open the PowerPoint presentation: Click File > Open and select the required presentation.
2. Open the Font dialog box: Click the Home tab and then select the small arrow next to the font size box.
3. Select the font size: In the Font dialog box, select the desired size in the Size dropdown.
4. Set the default font style: Select the font style from the Font dropdown.
5. Set the font as the default: Check the box next to Set as Default.
6. Confirm: Click the OK button.