Relevant to Blender v2.31
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 5.1, “The rotation point selection buttons”). 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, and so on), you can lock the scaling process to one of the global coordinate axes, just as you would when moving objects. To do so, enter scale mode, move the mouse a bit in the direction of the axis you want to scale, then press MMB. To return to uniform scaling, press MMB again. You will see the scaling factors in the header of the 3D window.
Again all considerations on constraining to given axis made for Grabbing still holds, as well as those on numerical input.
Here again CTRL switches to snap mode, with discrete scaling at 0.1 steps. Press SHIFT for fine tuning. The scaling of selected objects can be reset to the default value by pressing ALT-S.
Mirroring objects is a different application of the scale tool. Mirroring is effectively nothing but scaling with a negative factor in one direction. To mirror in the direction of the X or Y axes, press SKEY to go to scaling mode, then NKEY to switch to numeric input. Select the desired coordinates and enter '-1' as scaling factor.
You can also enter scale mode by drawing a V-shaped line while holding down LMB.