Wide angle, that is.
Generally, the smaller the room, the wider you'll want your lens field of view.
I shoot small rooms with a 14mm which captures nearly the entire room when I set up in a corner or doorway.
This can make a cramped room or even a closet look spacious, a regular bed into a king-size mattress, or this bathroom (right) palatial.
Keep the film plane vertical to avoid warping straight lines.
There is some distortion with such a lens (objects in the corners are stretched), so I switch to 20mm or a wide-to-normal zoom for larger rooms.
Those shooting with "cropping" digital bodies are likely to find small rooms a tough challenge.
Consider doing composite panoramas -- stitching is easy with digital and you avoid the cost of an expensive wide-angle lens.
Be aware there is a pitfall of using wide angle for every shot.
In doing so you might ignore interesting details: the unique character of some interiors can be found in smaller areas or objects, or even close-ups.
And viewing a portfolio with 100% wide angle and no variety can be rather tiresome.
Solution B: Move Furniture
Don't be afraid to rearrange in order to show a room to best effect... if you have time and some help.
Small rooms sometimes open up and look larger if some furniture items are removed.
You may also want to move furniture to emphasize positive features, cover up utilities, hide defects,
or to make room for your camera to set up in the best vantage spot.
Solution C: Add Light
See The Room Looks Dark for solutions.
Often having plenty of light makes a small room look larger, whereas poor light can shrink a large room.
