Pages using a common Master Page can be of varying lengths, longer than the MP.  Sometimes a background graphic is wanted to display the full length of all those associated pages.

Master Page A of this site is 800 pix high.
The HOME page is 900 pix high.  
Contact Us is 1000 pix high.
The red line graphic is 1100 pix high.

Using the Straight Line tool, I made the red line on the left 1100 pix high and placed it at the bottom on the MP and right click > Arrange > Attach to Bottom.

The red line extends onto the pasteboard both above and below the workspace.  The red line displays correctly on associated pages.



And, Andy H says.....
Zoom right out, click both lines using the shift key, this will bring up a centre node between them (along with the group symbol but there is no need to group), drag the node downwards this will eliminate any possibility of rotation providing there is no rotation on the original lines, and lengthen both lines at the same time, an alternative would be to select a line and drag the bottom node whilst holding down the shift key.

Serif forum link  Selecting lines to extend them