The length of the string is the number of characters in the string. Why is that value stored in a char type? It should be an int type.
I'd start with sending the entire readString to the LCD to confirm that the data is being read correctly, when readString contains msg.
Then, use substring (with one argument) to extract the first (or last, whichever it returns) n characters, and send them to the LCD, instead.
Then, use substring (with two arguments) to lop off both ends.