Exposition: the PLC-5 and SLC-500 operating systems had a fixed-length String datatype that was 82 characters long. So for compatibility, the ControlLogix default STRING datatype consists of a .LEN (4-byte DINT) value and a SINT[82] array of bytes.
You cannot change the fixed length of the default STRING datatype.
But you can create a custom-length STRING formatted datatype in Studio 5000 of any length you wish. It will have a .LEN element and a fixed number of characters for .DATA[x].
Packing arrays of strings in to UDTs like Shrivsrikakolum is doing can get complicated and use a lot of memory, but that's an extended discussion.