Maybe this helps:

http://docs.unity3d.com/ScriptReference/Mesh-bounds.html

Attach this sctript on the plane

 using UnityEngine;
 using System.Collections;
 
 public class GetBounds : MonoBehaviour 
 {
     Vector3 targetPoint = Vector3.zero;
 
     public void OnDrawGizmos()
     {
         Mesh mesh = GetComponent<MeshFilter>().mesh;
         Bounds bounds = mesh.bounds;
         
         // not affected by scale or rotation
         Debug.Log ("x: " + bounds.size.x +" y: "+ bounds.size.y + " z: " + bounds.size.z);
         
         // next you can simply calculate the start point of the ray
         Vector3 edgeA = transform.position + new Vector3(bounds.size.x * transform.localScale.x / 2, 0, 0);
         Vector3 edgeB = transform.position - new Vector3(bounds.size.x * transform.localScale.x / 2, 0, 0);
         Vector3 edgeC = transform.position + new Vector3(0, 0, bounds.size.z * transform.localScale.y / 2);
         Vector3 edgeD = transform.position - new Vector3(0, 0, bounds.size.z * transform.localScale.y / 2);
 
         // next if plane is rotated you must calculate position for this rotation...
         edgeA = RotatePointAroundPivot(edgeA, transform.position, transform.localRotation.eulerAngles);
         edgeB = RotatePointAroundPivot(edgeB, transform.position, transform.localRotation.eulerAngles);
         edgeC = RotatePointAroundPivot(edgeC, transform.position, transform.localRotation.eulerAngles);
         edgeD = RotatePointAroundPivot(edgeD, transform.position, transform.localRotation.eulerAngles);
 
         Gizmos.color = Color.red;
 
         Gizmos.DrawLine(edgeA, targetPoint);
         Gizmos.DrawLine(edgeB, targetPoint);
         Gizmos.DrawLine(edgeC, targetPoint);
         Gizmos.DrawLine(edgeD, targetPoint);
     }
 
     private Vector3 RotatePointAroundPivot(Vector3 point, Vector3 pivot, Vector3 angles)
     {
         Vector3 dir = point - pivot;             // get point direction relative to pivot
         dir = Quaternion.Euler(angles) * dir;     // rotate it
         point = dir + pivot;                     // calculate rotated point
         return point;                             // return it
     }
 }
 

It looks like this:

http://s13.postimg.org/h1cz5vfyf/bounds.jpg