The default zoom level set for PowerPoint slides in normal view is 88%. But you can change the zoom level using the option provided as part of the view menu or using the slider control in status bar.
Click view menu then navigate to Zoom section and click Zoon option.
Select your preferred zoom level then click the Ok button to apply the zoom setting to PowerPoint View.
If you want to revert to the original Zoom value then click Fit to Window option available under Zoom section.
Zoom Settings in Status bar
If you want to quickly change the change the zoom level then you can use slider control available as part of status bar. Similarly to revert the changes, you can use the Fit to Windows option available next to the slider control.