They average 2.5 to 3.5 ft and with sometimes males reaching 4 feet. It depends on where they were collected in the wild plus captivity is not the wild. They deal with alot different environmental changes that effect their growth (dry season, wet season)

The problem with most savannahs in captivity is that get into homes of people with limited experience as they are cheap and are kept in the wrong conditions and they are being fed the wrong diet. In the wild savannahs mostly eat insects with the occasional "mouse" say. You can fed rodents but i personally get a colony of Madagascar hissing roaches or dubia roaches going as they make excellent feeders
With low basking temps they cant digest food properly which leads to the food being stored as fat while high basking temps 125-130F min burns the fat and is used as energy as they will be active.
A good book to read is savannah monitor by Daniel Bennett and Ravi Thakoordyal it has tons of info in it and is one of the BETTER books out their with all good info.

Bryan