Open a new slides presentation.
Go to your Drive at drive.google.com and click the blue New button to open a new Slides presentation.
Once you've opened a new presentation, go to File > Page setup.
Change the size of the slides.
When the page setup box appears, click Widescreen 16:9.
Select Custom and enter the desired size of the slide. Be sure to select the appropriate unit, too (inches, centimeters, points, or pixels).
The slides will automatically resize. Now, you can add images, text, and more to the slides.