Varanus acanthurus (a.ka. the 'ackie') is a great starter monitor and there are a number of smaller ones as well, such as Varanus gilleni. If you want to go a bit larger than those, one of the sand monitors, such as Varanus gouldii or the desert sand monitor Varanus gouldii flavirufus. A bit bigger than those is the Spencer's monitor, Varanus spenceri. The larger species need a huge amount of space, though. One of the best things about animals the size of Varanus acanthurus is that they have all of the same attitude and behaviours of the larger monitors, but you can house a pair in a much smaller enclosure than the larger species and have a go at breeding them.