To change the size of objects, press SKEY. As in grab mode and rotate mode, scale the objects by moving the mouse, confirm with LMB or ENTER and cancel with RMB or ESC.
Scaling in 3D space requires a center point. This point is defined with the same buttons as the axis' supporting point for rotation (Figure 1 in the Section called Rotating objects). If you increase the size of the object, all points are moved away from the selected center point; if you decrease it, all points move towards this point.
By default, the selected objects are scaled uniformly in all directions. To change the proportions (make the object longer, broader, etc.), the scaling process can be locked to one of the global coordinate axes, in the same way as when moving objects: Enter scale mode, move the mouse a bit in the direction of the axis you want to scale, and press MMB. To change back to uniform scaling, press MMB again. You will see the scaling factors in the header of the 3D window.
A different application of the scale tool is mirroring objects, which is effectively nothing but a scaling with a negative factor in one direction. To mirror in the direction of the screen X or Y axes, press XKEY or YKEY, respectively, during scale mode. If you want a precise mirroring, make sure you don't move the mouse before confirming the scaling with LMB or ENTER.
Here again CTRL switches to snap mode, with discrete scaling factor at 0.1 steps, while SHIFT allows fine tuning.
An alternative way to enter scale mode is to draw a V-shaped line while holding down LMB. The scaling of selected objects can be reset to the default value by pressing ALT-S.