Selected 2D graphic elements show ‘grips’ at the ends of lines and on the rectangles surrounding them, which can be used to position or change the dimensions of the element.
Select one of the ‘grip’ points with a left mouse-click. For a line you can now reposition the point, or in the case of a surrounding rectangle you can change the size of the complete element. A second mouse-click terminates the operation.
Note: If you keep the ‘Shift’ key pressed while repositioning a point then the previous direction of a line remains fixed, and rectangles are only changed proportionally, i.e. the width-to-height ratio is retained. For lines you can thus select a point away from a line as a reference point for the new length of the line.