Not every platform contains uses 1:1 for their product ads. You can adjust the size of your artboard to best fit the platform you'd like to run on!
Change Placement Size
In the upper-right of the canvas, you will see the size of your current design. Click on the size and select the size from the platform in which you intend to run your DPA.
Now that you've adjust the size of your artboard, you can begin designing for the platform you want to run on.
One Placement Per Design
Each design can only hold one placement size. If you want to use the same feed design for a different platform, simply clone the design and change the placement size in the newly cloned feed design.