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 here.
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.