Not a true cactus but a succulent (despite its name), the pencil cactus can grow up to 30 feet high in the wild and well over 6 feet high indoors under the right conditions. (We've seen at least one indoor pencil cactus that was more like 15 feet long in one of our customers' houses!)
Bright, indirect light to medium light. Water when soil is totally dry. (Every 4 weeks, depending on conditions.) Sap is aggravating to skin; wear gloves when handing this plant and wash sap off skin after contact. Cactus soil.