To change the slide size and set it as default in Microsoft PowerPoint 2013, you can follow these steps:

1. Open PowerPoint 2013 and navigate to the “Design” tab in the ribbon at the top of the screen.
2. In the “Customize” group, click on the “Slide Size” button. It will open a dropdown menu with options for different slide sizes.
3. From the dropdown menu, select “Custom Slide Size.” This will open the “Slide Size” dialog box.
4. In the “Slide Size” dialog box, you can choose from different preset slide sizes or specify your own custom slide size. You can set the dimensions in inches, centimeters, or pixels.
5. Once you have selected the desired slide size, click on the “OK” button to apply the changes.
6. Now, to set the chosen slide size as the default for all future presentations, go to the “Design” tab again and click on the “Slide Size” button in the “Customize” group.
7. This time, select “Set as Default” from the dropdown menu. A confirmation dialog box will appear.
8. In the confirmation dialog box, click on the “Yes” button to confirm that you want to set the selected slide size as the default for all new presentations.

After following these steps, any new PowerPoint presentation you create in Microsoft PowerPoint 2013 will have the slide size you specified as the default.