Scale Object from Specific Side

To resize an object from a specific side:

Select an object.
Click the any face control point and hold down the mouse button, then press Cmd/Ctrl+Opt/Alt.
Drag right to enlarge it or left to shrink the object.
The object resizes in all directions from the center of the face opposite the selected control point. For example, if you select the Y control point on the top of the object, the object will scale up and out, but not down.
Make sure to select the control point and hold the mouse down before pressing Cmd/Ctrl+Opt/Alt. If you press Cmd/Ctrl+Opt/Alt first, nothing will happen.

To show some examples, below is a cube that has been moved so that the edges line up with the ground plane grid:

Select the X control point nearest the Director's View, press Cmd/Ctrl+Opt/Alt and drag right.
Notice how it resizes on the Y and Z-axes in both directions, but only to the right on the X-axis. The arrow shows the side that doesn't move.
Select the Z control point on the opposite side of the cube, press Cmd/Ctrl+Opt/Alt and drag right.
Notice how it resizes on the Y and X-axes in both directions, but only to the back on the Z-axis. The arrow shows the side that doesn't move.
Once more, select the Y control point on the top side of the cube, press Cmd/Ctrl+Opt/Alt and drag right.
Again, notice how it resizes on the X and Z-axes in both directions, but only up on the Y-axis. The bottom of the cube stays firmly on the ground plane.