How to find length/size of a key in a MultivaluedMap?

MultivaluedMap map= new MultivaluedMapImpl();
map.add("Accept-Encoding", "compress;q=0.5");
map.add("Accept-Encoding", "gzip;q=1.1");
map.add("Accept-Encoding", "gzip;q=1.2");
map.add("Accept-Encoding", "gzip;q=1.3");

How can I find the size of the key "Accept-Encoding"?