I was asked by a client yesterday about whether she could change the default size for text within text boxes. In the most up-to-date versions of PowerPoint, the default font is Calibri, size 18. However, many companies expect employees to use particular fonts for a presentation.
To change font for all text boxes, first create a text box by clicking Insert, then in the Text group, click Text Box; alternatively, to create your text box, click the Text Box button in the Drawing group on the Home tab.
Then type some text with the font type, size, colour, etc. that you require as the default for your text box font.
Now click on the textbox, right-click and from the shortcut menu that appears, click Set as Default Text Box.
The same font formatting will then be applied each time you create a new text box.