You'll need at least a 24x18 tank bearing in mind you'll need 1 moist hide, 1 warm hide and a cool hide plus water bowl, food bowl and calci bowl and decoration. I got mine brand new from here .VIVEXOTIC VX24 24" VIVARIUM IN BEECH FOR REPTILES ECT on eBay (end time 25-Aug-09 13:11:04 BST)
This is enough space for one leo. You also need to bear in mind how you will heat the tank
Alternatively, a glass tank will be ok too as long as it holds the right temps and has some form of vent so that there will be no condensation from the water evaporating.
If you're planning on getting more than one, two males will fight, a male and female will possibly breed and two females should be ok but they may accidently lose their tails. I'd say minimum for 2 leo's would be a 36x24 as you'd need to double the amount of hides. Also, leo's are incubated differently so that they end up a certain sex but it is not 100% guaranteed and it's very hard to sex young leo's in my experience.
Hope this is some help to you and post pics of when you end up with!