The size of the extracted value in bytes when will depend on the encoding you set up for the export.
To check what encoding you are using open your Document Definition in Document Definition Editor, go to the Export Settings, choose your export profile and open the options of the Data format.
If you are using the custom export or database export, you can check your database encoding settings or specify the encoding in the script.
As an example, the following script lets you store the size in bytes in the UTF8 encoding of the value of the text field Field1 in the howManyBytes variable.
var inputString = Context.Field("Field1").Value.ToString();
var howManyBytes = Encoding.UTF8.GetByteCount(inputString);
You can check the size for other encodings replacing the UTF8 with as an example Unicode or BigEndianUnicode.