Your designs (both images and videos) can be resized using the resize option in the editor. The resize option can be found at the top of the blue bar, beside the save option. 

You’ll see a list of pre-set sizes in the resize dialog. Some of the most commonly used layouts are: A4, flyer, poster, Facebook cover photo and square(1:1).

PosterMyWall offers over 60+ sizes for all kinds of marketing purposes, for both offline and online media. If you do not find the size you're looking for, try 'Custom Dimensions', which is available at the bottom of the sizes list.  You can set custom dimensions in pixels, inches and millimeters.

Select a size and then click ‘resize’ to instantly resize your design canvas. You may have to adjust design elements after resizing a design.

Note: If you pick multiple designs for resizing, copies of your design will open in new tabs, resized and ready to customize in the editor. The original design will remain unchanged.  

To learn more about auto resizing for multiple sizes, watch this video.