Make sure that you've enabled editing on the document, before getting started:

When you’re in PowerPoint and have the file open:

1. Click Over to the Design Tab
2. Select Slide Size
3. Select Custom Slide Size

Once you’re on that menu you can adjust the Width and Height. Be aware that adjusting the dimension to standard sizes such as 4x6, 9x6, etc. will make the file resolution low and will print low quality so you won’t want to put that in there!

Instead - you’ll have to put the same ratio in just larger numbers. Here are some standard postcard sizes in inches and their high quality equivalents to resize to in PowerPoint:
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8.5 x 6 = 41.708 x 29.792 in (Our Standard Postcard Size)
9 x 6 = 44.161 x 29.441

4 x 6 = 19.627 x 29.441

4.25 x 5.5 = 20.854 x 26.988

If you have a different size, here is the Aspect Ratio calculator we used! https://calculateaspectratio.com/

For more pointers on editing in PowerPoint, check out our PowerPoint Tutorial in Tech Tools and Training!