Exanthematicus (Bosc/Savanah) - 3 feet, give or take a little. However, Daniel Bennet wrote of a study done of a locale in Ghana of 16 adults where the average length was 130cm. I'd imagine that lizards that size would be extemely rare though.
Ionides (Black Throat) - 5-6 feet.
Albigularis (White Throat) - Reports of 213cm specimens, 190cm would be rare according to Daniel Bennet. Cape Banded White throats are smaller, closer to exanthematicus and very cool looking.
These three species are all found in Africa and are very similar. They are easily mislabled so depending on what you have, the lengths could vary. Exanthematicus are by far the most common though, and is what is pictured above (a very beautiful one too!)
they are awsome pets the more tame they get the more of a dog they are... 4 years ago i owned one a full grown one he use to sleep beside me great pets... i do have a pic of him but its in storeage..i go get it and post on here soon...