The scale tool in SketchUp (keyboard shortcut S) allows you to quickly and accurately change overall dimensions of shapes. With an item selected you can activate the scale tool to scale that object. You will be presented with a yellow boundary box with green hand grabs on the endpoints and midpoints of the perimeter as well as the center points of the faces.
From there you can select any one of the green hand grabs. Once selected they will turn red and display a connecting line that represents the direction of the scale. When a component is selected the scale tool will only modify its overall dimensions and not is actual shape. When lines and planes are selected in model space you can modify not only the overall dimensions but also the shapes of faces.