How do I get value decimal follow block byte item split in byte array ?

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C#

C#3.5

Hi, I have a byte value array containing elements, I want to get the value by block and convert to decimal, element block value in predefined. Assume, I have this code as below.

var _bytes= new byte[]{0, 12, 0, 6, 19, 136};
var _size = 3; //byte with 3 item is decimal
var _num = 2 ; //length of block define in array byte
var _result = "";
for (int i = 0; i < _size; i++)
{
   //get value decimal in byte array follow the block define 
   //var _id = BitConverter.ToInt16(_bytes[i] * 2, 2);
   _result += _id + " ";
}
//the _result should be "12 6 5000"
//0 12 is decimal 12
//0 6 is decimal 6
//19 136 is decimal 5000

What I have tried:

try to convert but result not correct