convert textbox character size in to kb in c#

Are you saying you have something like this:

string s = "48656c6c6f2c20776f726c6421";

and you want these values as a byte array? Then:

public IEnumerable GetBytesFromByteString(string s) {

    for (int index = 0; index < s.Length; index += 2) {

        yield return Convert.ToByte(s.Substring(index, 2), 16);

    }

}

Usage:

string s = "48656c6c6f2c20776f726c6421";

var bytes = GetBytesFromByteString(s).ToArray();

Note that the output of

Console.WriteLine(System.Text.ASCIIEncoding.ASCII.GetString(bytes));

is

Hello, world!