Click the ‘Size’ button on the top navbar. It’s showing the size of the design you are currently working on.
Then select a dimension to resize your design. You can either convert it to a custom size or a pre-defined size.
Finally, choose whether you want to override your existing graphic or open as a new graphic. If you choose to open it as a new design, the current design will be saved in “Your Design” section to be used later
And that’s it. Your graphic has now been converted to a new size! Note that sometimes you may need to make adjustments if the aspect ratio has changed substantially.