Resizing designs:
On the menu bar above the editor, click Resize.
Enter custom dimensions, or pick from the preset design size options. Hover your cursor over them to see their dimensions. You can select more than one size.
To resize the current design, click Resize.
To keep your current design unchanged and create a new resized version of it instead, click Copy & resize.
Make sure to select a new design size from the options, or to enter new dimensions. Otherwise, the Resize and Copy & resize buttons will be grayed out.