Dan Flavin's room-size installations are some of his most ambitious and immersive works, often specifically designed for particular museums or exhibitions. These installations used the architectural space as a canvas, transforming the environment with color and light.

Flavin's room-size installations often included arrays of fluorescent tubes in various lengths, colors, and configurations. The fixtures were arranged to fill the space and interact with its architectural features, casting colored light onto the walls, floor, and ceiling. The light from these installations would reflect off the surfaces of the room, creating an immersive environment of color and light.

One notable example of a room-size installation by Flavin is his work for the Dia Art Foundation in Bridgehampton, New York. Titled "Dan Flavin: in daylight or cool white," the exhibition featured a series of rooms filled with fluorescent light installations. Flavin used different combinations of lights in each room, creating a progression of color and light effects as visitors moved through the space.

Another significant installation is "untitled (Marfa project)" in Marfa, Texas. Commissioned by the Dia Art Foundation, it includes several buildings filled with large-scale light installations, many of them room-size.

These room-size installations are powerful demonstrations of Flavin's innovative use of light as an artistic medium. By transforming entire rooms with his light installations, Flavin blurred the boundaries between the artwork and the space in which it was displayed, creating immersive environments that directly engage the viewer's perception of light, color, and space.