From BlenderWiki

Copy Scale Constraint

Description

The Copy Scale constraint forces its owner to have the same scale as its target.

Here we talk of scale, not of size! Indeed, you can have two objects, one really bigger than the other, and yet both of them having the same scale… This is also true with bones: in Pose mode, they all have a unitary scale when they are in rest position, whatever being there visual length…

Options

Targets

This constraint uses one target, and is not functional (red state) when it has none.

Offset

When enabled, this control allows the owner to be scaled (using its current transform properties), relatively to its target’s scale.

X, Y, Z

These buttons control along which axes the scale is constrained – by default, it is along the three ones.

CSpace

This constraint allows you to chose in which space evaluate its owner’s and target’s transform properties.

Examples

Objects

Copy Scale between objects example.

No constraint.

Owner copying scale of Target_1, without offset.

Owner copying scale of Target_1, with offset.

A Copy Scale constraint applied to Owner empty, using Target_1 empty as target. In the third picture, the Offset option is enabled, and hence the owner‘s scale is offset from its target’s one. Without the Offset option, the constrained object cannot be scaled, it is stuck to its target’s scale. With the Offset option, it can be scaled, relatively to its target…

Armature’s Bones

The green constrained bone copies the scale of the blue target one…