The best colour for opening up a room is bright white, so try painting this on all the walls. Neutral colours such as this give the impression that the walls are further away than they actually are.
If you want the room to be slightly warmer, try adding a few tints of your favourite colour.
If so much white really isn't your thing, there are alternatives such as beige or light grey, which also work well. You could also try yellow, which is great for creating space due to it's ability to reflect light, just like white.
Blues, and grey-blues on the walls allow all the other colours in the room to stand out on the neutral backdrop, handily shifting the focus to the furnishings rather than the room itself.
Always try and keep to one or two main colours that flow from room to room, and certainly don't use more than three.
Pastels, and light natural tones are excellent for creating an airy feeling in small bedrooms,bathrooms, and hallways.
You can bring the walls and woodwork together by painting both in colours from the same family, creating a light expanse. Use matte on the walls, and gloss or eggshell on the wood, so that it is not too flat.
Stripes can completely change the shape of a room. Narrow vertical stripes in white or soft pastel will make a low ceiling seem much higher. You can of course simply paper the walls, or be more adventures and paint the stripes on.
To make the room look longer, simply turn the stripes horizontal, and only use 2 or 3. This works especially well with a light coloured backing.
It's not only the wall colours that affect the sense of space, but the floors too. It is best to use one carpet colour throughout the house. Try painting the skirting board in a similar tone to break down the boundary where wall meets floor.
In a small bedroom, use a solid neutral carpet, or be a little more adventurous and use one with horizontal stripes to widen it.
