Tarantulas don’t need or want large cages – they are used to tight burrows and retreats and can get lost in a large cage, unable to find their food and water. You also don’t need a very high cage; it would encourage a tarantula to climb up to comers where it could fall long distances, which is dangerous.
For virtually all common tarantulas (except the very largest, such as king baboons and Goliath bird-eaters), provide a terrarium roughly a foot (30.5 cm) long on each side, and at most a foot high. Many common tarantulas will do well in a smaller container, but it is difficult to provide a water dish as well as a cover for the retreat in such a tight space.
At one time, tarantulas commonly were kept in 1-gallon (3.8 liters) screw-top jars with a few holes punched in the top for air movement. When given about 2 to 4 inches (5 to10 cm) of substrate, the spiders did well and lived long lives. Today, the tendency is to use plastic “critter” boxes (the types that come with slotted plastic tops), small all-glass aquaria with tight-fitting lids, or plastic or glass cages custom-made to house tarantulas. You probably soon will want to expand your collection beyond your first purchase, so consider using a standard size cage so several will fit evenly on a shelf, saving space and making care easier.
Just putting a sheet of glass on top with a brick for weight probably won’t work, as the spider will gradually shift the glass. Try to avoid very fine mesh (less than 1/4 inch [6.4 millimeters]) for the lid, as a tarantula can catch a fang in it and suffer a break; it may take several molts for the fang to regenerate. The entire top of the terrarium should either be made of screen or slotted to allow good air movement. If you need to increase humidity in the cage, place a sheet of flexible plastic (like that used to insulate windows) between the lid and the frame of the cage. By increasing or decreasing coverage of the top by the plastic, you should be able to control the humidity fairly well. Though many books recommend mesh inserts at the top and bottom of facing sides of the cage to increase air circulation, I don’t feel this is necessary or practical—glass and plastic are hard to cut, mesh is hard to safely fix to the glass or plastic, and the tarantula can catch a fang in fine mesh. Side mesh panels also cause rapid loss of humidity from the cage.
It often is recommended that burrowing tarantulas be kept in shallow terraria (just a bit higher than the highest point they can reach when standing upright) to prevent injuries or death due to falls from heights greater than a foot (30.5 cm). Accidents have indeed happened in taller terraria, but generally they are not dangerous; the exception is for heavy tarantulas, whose weight increases the likelihood of injuries or death from falls.