The largest known cicada is the Megapomponia imperatoria (Westwood, 1842) of south-east Asia. The specimen in the photo below was captured in Malaysia and it’s wingspan measured 20 centimters/7.9 inches. Other species might be larger in terms of weight, but I’m not sure.
Other very large cicadas include the Bear Cicada of Japan (Cryptotympana facialis), and Tacua speciosa of south-east Asia.
The largest cicada in North America is the Megatibicen auletes:
These very large cicadas are loud, but not the loudest. Learn about loud cicadas.