A 3D object like KinematicBody has a Vector3 as scale (you also see 3 values in the transform scale section of the Editor). This is common for all scales.
KinematicBody is derived from Spatial. Therefore it has all properties of Spatial.
If your node is named "KinematicBody1" then this example should set the scale to 10 in all dimensions:
$KinematicBody1.scale=Vector3(10.0, 10.0, 10.0)