Scale Along a Single Axis

To resize an object along a single axis interactively:

Select an object.
Drag one of the control points at the center of a bounding box face. As you pass your cursor over the face, the cursor changes to an X, Y or Z to indicate the axis you're resizing along.
Drag right to enlarge it or left to shrink the object.

The object will scale along the axis of the selected control point.