Male Argentine B&W tegus and red tegus usually get to be around 4 feet long (the Paraguay or Chacoan White-Headed morphs tends to be about half a foot longer). Blue tegus usually grow to between 3 feet and 3.5 feet in length. Females of all tend to be smaller, usually staying between 3 and 3-1/2 feet in length. There is a tegu forum where you can ask more questions about them (as someone else pointed out, they are not monitors). If you're new to tegus, you will probably want to stay with one of the above types. Golden tegus (the kind most often found in pet stores) have a reputation for being much harder to calm down (and watching my hatchling tegu take crickets, I don't want to be on the bad side of a large tegu!). My personal recommendation is to acquire a captive-bred hatchling (Ron St. Pierre/Blue Tegu and Agama International are both well-known breeders).
If you are indeed looking for a monitor in that size range, you're probably mostly interested in the dwarf monitors (acanthurus or "ackies", timors, etc.). Ackies seem to be the most highly recommended of these monitors. I have one, and he's a wonderful little clown, truly fun to watch. I also have one of Ron St. Pierre's Chacoan tegu hatchlings, and he's been a lot of fun, too. He's very bold and curious about everything, and after I'd had him only a couple of days, he would come over to tongue-flick me when I came to his cage, and tolerated my touching him and playing with his tail.