If something has plenty of space, it's roomy. Your enormous dorm room is roomy, and the sweater that hangs halfway down your thighs is also roomy.
Roomy things are spacious or sizable — they're big. A roomy old house has enormous rooms and plenty of them, and roomy clothing is the apparel that's loose and comfortable. Sometimes, roomy is used as a noun, which is also spelled roomie, as shorthand for roommate. The seventeenth century version of the adjective was roomsome.