Change the length of a line and keep same width?
Solution 1:
On the far right side of the Tool Controls Bar, you'll find four buttons (to the right of the "Affect:" label; see the screenshot below). You can use those to control which aspects of an object are affected by, e.g. resizing operations.
Solution 2:
Select the Edit Paths by Nodes tool and grab one end of the path. You can then move it around. If you're using freehand lines it's a bit more complicated but the principle is the same (you have to move multiple nodes).