550 feet is the record

The tallest known iceberg in the North Atlantic was 550 ft high, extending out of the water to almost the height of the Washington Monument. Seven-eights of an iceberg's volume is below sea level! The most dangerous icebergs are not the largest ones, however, since they are easily seen on ship radars. The more dangerous ones are the ones about the size of a ship. This iceberg was sighted in Melville Bay, Greenland from the ice breaker USCGC Eastwind by CDR Dinsmore in March 1957 in approximate position 75N, 67.5W.

Credit: U.S. Coast Guard, International Ice Patrol

For more info: http://www.uscg.mil/lantarea/iip/FAQ/faq11.html