Legacy Documentation: Version 4.3.0
Handles.ScaleSlider
static function ScaleSlider(scale: float, position: Vector3, direction: Vector3, rotation: Quaternion, size: float, snap: float): float;
static float ScaleSlider(float scale, Vector3 position, Vector3 direction, Quaternion rotation, float size, float snap);
static def ScaleSlider(scale as float, position as Vector3, direction as Vector3, rotation as Quaternion, size as float, snap as float) as float
Parameters

scale The value the user can modify.
position The position of the handle.
direction The direction of the handle.
rotation The rotation of whole object.
size The size of the handle.
snap The new value after the user has modified it.

Note: Use HandleUtility.GetHandleSize where you might want to have constant screen-sized handles.
Description

Make a directional scale slider.


Scale slider handle in the Scene View.
	// Increase/decrease a value just by moving the Scale slider Handle
	
	
	@CustomEditor (FreeScale)
	class FreeScaleHandleJS extends Editor {
		function OnSceneGUI () {
	        target.val = Handles.ScaleSlider(target.val, 
	        				target.transform.position, 
	        				Vector3.up, 
	        				Quaternion.identity, 
	        				5, 
	        				HandleUtility.GetHandleSize(target.transform.position));
	        if (GUI.changed)
	            EditorUtility.SetDirty (target);
	    }
	}
And the script attached to this handle:
	// FreeScale.js
	@script ExecuteInEditMode()
	
	var val : float = 1;
	
	function Update () {
		Debug.Log("Value tied to the slider: " + val);
	}