Create a script called "ResizableObject" and add it to your object.

using UnityEngine;
using System.Collections;

public class ResizableObject : MonoBehaviour 
{

}

And Then create a script called "InputHandler" and create a new gameObject and add this script to it.

using UnityEngine;
using System.Collections;

public class InputHandler : MonoBehaviour 
{
 private Ray m_Ray;
 private RaycastHit m_RayCastHit;
 private ResizableObject m_CurrentObject;
 private Vector3 m_LastMousePos;
 private float m_DeltaTime;
 private bool m_AnimateScale;
 private Vector3 m_StartScale;
 private float m_ScaleFactor;

 void Update () 
 {
 if(Input.GetMouseButtonDown(0))
 {
 m_LastMousePos = Input.mousePosition;
 m_Ray = Camera.mainCamera.ScreenPointToRay(Input.mousePosition);
 if(Physics.Raycast(m_Ray.origin, m_Ray.direction, out m_RayCastHit, Mathf.Infinity))
 {
 ResizableObject obj = m_RayCastHit.collider.gameObject.GetComponent<ResizableObject>();
 if(obj)
 {
 m_CurrentObject = obj;
 m_StartScale = obj.transform.localScale;

 }
 }
 }

 if(Input.GetMouseButton(0))
 {
 Vector3 deltaPosition = Input.mousePosition - m_LastMousePos;
 if(deltaPosition.magnitude > 1f)
 {
 if(m_CurrentObject && !m_AnimateScale)
 {
 m_ScaleFactor = deltaPosition.magnitude;
 m_AnimateScale = true;
 m_DeltaTime = 0f;
 }
 }
 m_LastMousePos = Input.mousePosition;
 }

 if(m_AnimateScale && m_DeltaTime < 1f)
 {
 m_DeltaTime += Time.deltaTime;
 if(m_CurrentObject)
 {
 m_CurrentObject.transform.localScale = Vector3.Lerp(m_CurrentObject.transform.localScale, m_StartScale * m_ScaleFactor, m_DeltaTime);
 }
 }
 else
 {
 m_AnimateScale = false;
 m_DeltaTime = 0f;
 }
 }
}

This just scales-up the object not scale-down. It's relatively easy to implement that if you want.