radius of an adult plant - HELP

im trying to design a closet operation and the dimensions are 18" deep, 72" wide, 66" tall. I'm trying to figure out if 18" is wide enough to let a plant grow properly. any thoughts? I would probably have 3-5 in a line under a 400w hood/ballast and then use the rest of the space for filter etc.

plants will adapt to any space. if u had unlimited space the plants will just bush like crazy. in your case the plants will adjust to the space and just grow up. since u dont have a lot of room try pruning some smaller leads so u have more top colas since light mite not reach to the lower levels due to cramped space.