There are three craters which could qualify as the biggest, because the sizes of many craters are uncertain. One of the biggest is the Sudbury crater in Ontario, Canada, which is 200-250 km in diameter. There are a couple other large craters as well. The crater in Vredefort, South Africa is 150-300 km. (It's hard to measure the exact sizes of these craters because they are old and have been mostly eroded. So when I look up the sizes I get many different numbers, which I just listed here as a range.) The Chicxulub crater (which is the crater that formed at the end of the Cretaceous period and wiped out the dinosaurs) is about 175 km across. Wikipedia contains a good list of craters on the Earth:
Updated on July 18, 2015.