https://answers.microsoft.com/en-us/msoffice/forum/all/default-font-for-ppt-objects/a081100d-76fd-4367-abe9-085c70966893
If you want to set the default font size or background color of text boxes or callouts:
- View - Slide Master - Insert Layout (This will create a custom layout.)
- Home. Create a textbox. Change the font size and background color.
- Right-click the text box and choose "Set as Default Text Box".
- Create a callout and change its font size or background color as you wish.
- Right-click and "Save as Default Shape".
- Slide master - Close Slide Master.
