Forget the old wives' tale that dark colours "shrink" a room — they don't, not in the way people think. What dark paint actually does is blur the boundaries of a small space. When walls, ceiling and woodwork all disappear into the same deep tone, your eye stops measuring the corners and the room reads as enveloping rather than cramped. That's why dark cloakrooms, snugs and box-room studies almost always work, mate.
So the real question isn't size — it's what the room is for and how much light it gets.
If it's a small room you want to feel calm, restful and intimate — a downstairs loo, a snug, a north-facing study — go dark and commit fully. Wrap the whole room, ceiling included, in something like Farrow & Ball Hague Blue or Inchyra Blue, or Little Greene's Dark Lead. Half-measures (dark walls, brilliant white ceiling) are what make a small room feel boxed in — the contrast draws a hard line round the space.
If instead you want the room to feel bigger and brighter — a small bedroom, a pokey hallway you pass through — go light, but go *soft* light, not stark white. A warm off-white like Farrow & Ball Pointing or a gentle calming neutral like Cornforth White reflects light without the clinical glare of a pure brilliant white, which can actually make small rooms feel cold and flat.
The "but what about my one small window" worry: a dark room with poor natural light becomes a glorious evening room — atmospheric under lamplight — but can feel gloomy at 10am. If you mostly use the space in daylight, lean light. If it's an evening room, dark wins every time.
Practical advice: whatever you choose, paint the woodwork and ceiling in the *same* colour family as the walls, not contrasting white. Continuity is the trick that makes small rooms feel intentional rather than squeezed. Test two large boards in the actual room and live with them for a couple of days before you commit.