Select the relevant border by clicking on the border itself or its name in the bottom left.
If you cannot select the Border, please see point 3 on our troubleshooting page.
Once selected, you will see a Red node in one corner of the border and 3 Blue nodes in the other corners (see image below).
To re-size the border, select one of the Blue nodes and stretch the border as required.
If your border is a Custom border, you will be able to change the aspect ratio, if it is not a custom border, the aspect ratio will stay the same.