You can resize active straight lines using any of the following methods:

• Select the Item tool and move the Arrow pointer over an item handle to display the Resizing pointer. Click and drag the handle to a new location to reduce or extend the length of the line.
• Choose Item > Modify (Command+M/Ctrl+M), then click the Line tab. Click the Mode drop-down menu to display the four mode options (see “Line modes for straight lines“). Choose Left Point, Midpoint, or Right Point to display a Length field. Enter values in the Length field to precisely change the length of a line, then click OK.
• Choose either Left Point, Midpoint, or Right Point from the Line Mode drop-down menu in the Classic or Space/Align tab of the Measurements palette to display the L (Length) field. To precisely change the length of a line, enter a value in the L field, then press Return/Enter.

You can resize any Bézier line by modifying the size of its bounding box. To do so, make sure Item > Edit > Shape is unchecked, and then resize the line as if it were a box.