A good estimate is to assume that half of the length is tail, however, Water monitors, Nile monitors, and tree monitors have longer tails.
Ackie monitors are great starter monitors usually only getting to 1 1/2 ft. to 2 ft. in length.
Savannah monitors are good, as long as you handle them from a young age, they will usually reach about 4 ft. in length.
I will make you a small list of monitors that stay relatively small, but it will not be in order, just what I can come up with off of the top of my head. Also, not all are going to be beginners, so please research it before making a decision. Any how, here is a list:
Peacock monitor- About 2ft.
Ridge tail/ Ackie monitor- 1 1/2-2ft.
Black tree monitor- 3-4ft.
Golden spotted- tree monitor- Around 3ft.
Short tailed monitor- About 10in.
Mangrove monitor- 3 1/2-4ft.
Peach throat monitor- About 3ft.
Green tree monitor- About 3ft.
Timor monitor- 2 1/2-4ft.
Black headed monitor- About 2 1/2ft.
Blue tree monitor- About 3ft.
Blue tailed monitor- About 4ft.