This article will help you to:
Adjust the dimensions of the reports exported to Google Slides
If you are trying to export a report which does not have the standard Google Slides dimensions (width: 960 pixels, height: 540 pixels) for the first time, here is what you need to know.
In Google Slides' top menu, click on "File" > "Page setup" > "Custom" > "Pixels" and copy-paste the dimensions of your report > Apply and you are done!
You will only need to do this once (the first time) for every report.
You may find your report's dimensions on the right-hand side of the report overview screen. Here is an example: