Childreni are nice, we have one. Ours lives in one of a bank of 4 enclosures, 3x2x2 ft. This bank was made from a kitchen cupboard, MY kitchen cupboard (not impressed)
hubby just adjusted shelves, added tracking for glass doors and wired it himself, even built our own bulb cages.
It really depends on what you want at the end of the day.
A neat functional enclosure can be cheap as you like or a very nice attractive enclosure that will compliment your furniture can be as expensive as you like too.
At minimum size for an adult childreni, (its my opinion) should be 3x2x2 feet
ofcourse you can go larger, but a childreni might freak out in a huge 6 ftx4x4x for example.
Unless your buying an adult, you will have ample time to decide about its eventual home, a hatchling can be kept in a breadbox size plastic tub (ventilated) with a small heatmat or heatcord under one third of the floor, a simple hide or two and small water bowl.
do a search here for 'enclosures' or/and 'tubs' or 'click clacks' you will no doubt find pics of all kinds to give you a good idea what you will need.