Monitor lizards should not be kept in tanks. Even dwarf monitors need at least 12 inches of soil to burrow into. I would suggest getting a cattle watering trough or building a custom enclosure.

I use a trough for my ackies with 12-14 in. of soil in the bottom. Here is a picture:

Ackie size also depends on what kind of ackie you buy. Red ackies are generally the biggest and can easily hit the 3' mark. This guy is close to 3' TL but as it was mentioned...most is tail.

Yellow ackies are a little smaller usually toping out at 18"-20". There are also alot of hybrid red/yellows around that usually fall in between the size range.

The price of reds varies from around $350-$250 depending on color and breeder. Yellows and hybrids are sold for around $200 each.

They are wonderful monitors to keep and definately 'better' than alot of other CH/WC monitors that are sold in the pet trade.

Research as much as possible before making your purchase. For starters, the pro-exotics site has some great info so I would sugggest reading it though once or twice.