Millipedes don't require a tall terrarium, but they do need a lot of floor space. You can house your pet millipede in a terrarium or fish tank, but it must be correctly sized. A millipede requires a container at least twice as long as its body, and twice as wide as its body. If you plan to keep more than one pet millipede in the same tank, size the terrarium according to the length of your longest millipede. A 10-gallon tank is usually large enough for a giant African millipede, the kind commonly sold in pet stores.
Though millipedes will spend most of their time on the ground in your terrarium, they can and will climb the sides at times. Be sure to cover your tank with a secure screen lid, and keep it locked in place to prevent your pet millipede from escaping.
Cover the bottom of your terrarium with 4-5 inches of potting soil, moist sphagnum moss, mulch, or unbleached paper towels. If you use potting soil, be sure it doesn't contain fertilizers or other additives that may harm your millipede.
Provide a place for your pet millipede to hide; a rock shelter or overturned piece of bark works well.
Add a shallow dish with water, and place stones in the dish so your millipede will not drown. You can also use a sponge in the dish, but this will need to be changed regularly because it can grow bacteria that may harm your millipede. If your tap water is chlorinated, let it sit for 24 hours before adding it to the water dish.
For more on keeping millipedes as pets, read my Guide to Caring for Pet Millipedes.
