According to Official Guinness Records,
The largest raindrops ever directly recorded measure a minimum of 8.6 mm (0.338 in) across. They have been detected on two occasions;September 1995 (Brazil) and July 1999 (Marshall Islands). The raindrops were imaged while falling by a laser instrument on board a research aircraft in studies by Professor Peter V. Hobbs and Arthur Rango (both USA), University of Washington (USA). Their results were published in Geophysical Research Letters on 13/07/2004.
For a complete list of 2004 records, please visit 2004 Guinness Records in Brazil.