C# how convert large HEX string to binary

I have a string with 14 characters . This is a hex represantation of 7bytes. I want to convert it to binary. I tried using Convert.ToString(Convert.ToInt32(hexstring, 16), 2); For small strings this works but for 14 characters it will not work because the result is too large. How can i manage this? Keep in mind that the output of the conversion should be a binary string with a lengeth of 56 characters (we must keep the leading zeros). (e.g. conversion of (byte)0x01 should yield "00000001" rather than "1")