Declare an array of length 2M, but I have to just keep 5K, how to get inside the real value?

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C#3.0

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C#4.0

  string[] splitString = client.Client.LocalEndPoint.ToString().Split(':'); 
            IPEndPoint local = new IPEndPoint(IPAddress.Parse(splitString[0]),Int32.Parse(splitString[1]));
            receiveUDPClient = new UdpClient(local);
            IPEndPoint remote = new IPEndPoint(IPAddress.Any, 0);
            int recAllLength = 0;
            while (true)
            {
              
                try
                {
                    byte[] receiveBytes= new Byte[1024 * 1024 * 2];
                   
                 byte[] arr=new byte[1024*5];
               
                   arr = receiveUDPClient.Receive(ref remote);
                 
                   Buffer.BlockCopy(arr, 0, receiveBytes, 0, arr.Length);
                   int recLength = receiveBytes.Length;//debugs says recLength=1024*1024*2
                  // MessageBox.Show(recLength.ToString()+","+arr.Length);
            while (arr.ToString() != "over")
                 {
                     arr= receiveUDPClient.Receive(ref remote);
                     MessageBox.Show(arr.Length.ToString());
                     if (arr.ToString().Contains("over,"))
                     {
                         string[] splitstring = arr.ToString().Split(',');
                         recAllLength = Int32.Parse(splitstring[1]);
//I want to get the real length,but actually it is the entire length of the array, not the length of valid data                      
                            break;
                     }
                     MessageBox.Show(recLength.ToString() + "," + arr.Length.ToString());
                     //debugs Exception,Array bounds because reclength=1024*1024*2;
                 Buffer.BlockCopy(arr, 0, receiveBytes,recLength,arr.Length );                
                   // arr = null;
                     recLength += arr.Length;
                }
                    MessageBox.Show(recLength.ToString());

Now receiveBytes inside length of the value of 5 * 1024, but in fact may be less than the length of 5 * 1024, if the loop receiver, will not get the true value, and how do I get the true length of the receiver after the arr?
debug says recLength=1024*2*1024,but in fact it can be less than 5*1024,how can I get the real reclength?