Shyam,
If target is text file use fieldLengthExceeded of StrictXml2PlainBean.
Adding StrictXml2PlainBean (XML to Text) in the Module Processor
If you want to cut length in message mapping create below UDF.
public String udf_trim(String in, int len, Container container) throws StreamTransformationException { if (in.length() > len) { return in.substring(0, len); } return in; }