Like Kristin said, some colors will make a room seem bigger, and some smaller. And she is right about the colors- blues undertones are cool colors and "recede", yellow undertones are warm colors, and they will "advance". If you stick with receding colors it will make it feel bigger.
The ceiling might look lower like 5's said, but if your room is fairly large it would probably be ok. Or you could do a trim or border around the top of the room and that might help too, something to make the transition to the ceiling so you don't have the sharp vertical lines breaking up space. I agree about the flooring, but you might just be able to get away with putting a dark throw rug down and that would help too, especially if you have dark furniture to anchor the room.
Anyways, I think it sounds really cute and I would go for it personally. I love dark, bold paint in rooms, so much more like a home than the beige I currently have on my walls.