GameObject size center of the screen

I'm downloading 3d model dynamically using Obj Reader and displaying in the scene but sometimes model showing too small and too big. I can't place in center of the screen for all the model. How to do this?

// duplicate the referenced image target newImageTarget3d = Instantiate (ImageTargetTemplate3Dload.gameObject) as GameObject;

         // enable the new result with the same ImageTargetBehaviour: ???
         ImageTracker tracker = TrackerManager.Instance.GetTracker<ImageTracker> ();
         ImageTargetBehaviour imageTargetBehaviour = (ImageTargetBehaviour)tracker.TargetFinder.EnableTracking 
             (targetSearchResult, newImageTarget3d);            
         
         
         StartCoroutine (seperateJsonLoad3D (mTargetMetadata, targetSearchResult.UniqueTargetId , imageTargetBehaviour ));

Obj Reader Library to get the model from server url:

 IEnumerator seperateJsonLoad3D ( string jsonURL, string targetID, ImageTargetBehaviour itb ){
         
         
         WWW www = new WWW (jsonURL);
         yield return www;
         
         jsonText = www.text;
         
         Debug.Log("3D Url load is "+ jsonURL);
         
         JSONObject json = JSONObject.Parse(jsonURL);
         
         type=json.GetString("type");
         url=json.GetString("link");
         modelShowGUIButton=true;
         
         
         
         if(type.Equals("3d")){
             
              modal = GameObject.Find (targetID);
             
         
             if (!modal) { //fetch model from the URL
                 
                 Debug.Log("3D !modal is "+ url);
                 
                 loadingText = (GUIText)GameObject.Find ("LoadingText").GetComponent(typeof(GUIText));
                 
                 
                 // loadingText = GameObject.Find("LoadingText").GetComponent(GUIText);
                 loadingText.enabled = true;
                 
                 var objData = ObjReader.use.ConvertFileAsync (url , true);
         
                 
                 while (!objData.isDone) {
                     loadingText.text = "Loading... " + (objData.progress*100).ToString("f0") + "%";
                     yield return null;
                 }
                 
 
                 
                 loadingText.enabled = false;
 
                 string modelName = objData.gameObjects[0].name;
                 
                 modal = GameObject.Find (modelName); // find the model reference
                 
                 modal.name = targetID; // change the model name to targetID !!!
                 
                 // store the original model somewhere safe!!
                 modal.transform.position = new Vector3 (modal.transform.position.x, 500f, modal.transform.position.z);
                 
             }
             
              modalClone = Instantiate( modal ) as GameObject; //clone the model 
             
             modalClone.transform.localScale += new Vector3(250,250,250);
 
             /modalClone.AddComponent(MeshCollider);
             mesh = modalClone.AddComponent<MeshCollider>();
 
             modalClone.transform.parent = itb.gameObject.transform; // pair the clone to the instantiated image target
 
         modalClone.transform.localPosition = new Vector3(0,0,0);
 
 
 
 
         }
         
         
         
     }