You'd never want a 6 foot plant indoors. It's not an efficient use of artificial light.
Plants grow as tall as you and their genetics let them. Most indoor growers will stay at about 3 feet and under.
A closet that is at least 6 feet tall will give you enough space for a 3 foot plant, a foot tall pot, and 2 feet to hang lights and ventilation. You will need to run an air-cooled hood with a large light in that space.
Pick strains that stay short and shrubby and train them so that they go into flower at less than 2 feet in height.