Most geysers anywhere in the world really aren't very big; however, there are a few that have been truly enormous. One of these is Steamboat Geyser in the Norris Basin of Yellowstone. It hasn't erupted for a year and a half (and has had dormancies as long as 50 years), but its major eruptions are truly a site to behold. One couple who was camping in the Norris parking lot was awakened in 2000 by what they thought was an earthquake. It turned out to be Steamboat erupting. It apparently is always obvious when a major eruption has occurred because of the rocks that are thrown around.
Impressive as Steamboat may be, there have been bigger and more violent geysers. Waimangu ("black water") in New Zealand erupted to heights of 1500 feet (Steamboat's very largest eruptions have been estimated at 380 feet). It only erupted from 1900 to 1904 when the geyser was destroyed by landslides. However, it has the dubious distinction of killing four people who were standing about 90 feet from the geyesrwhen a major eruption began in 1903.