We've all got rooms that feel a little too small, regardless of what size they actually are. It's far too easy to have a badly arranged, cluttered and mis-matched room, which leads to the visual constrained size appearance of the actual space. There are a number of simple things you can do to counteract this visual distraction, some involve changing colour palettes and patterns, and some involve the use of specific furniture and fittings to help create a visually larger room feel.
Try and use natural light where possible. Fundamentally this is your key source of light, which should be complemented and used as much possible. This complements almost all advice below, IE using mirrors to reflect light, use colour shades to reflect light. By keeping floors darker than the ceilings you can emphasise space, and by using trim paints in similar shades can really complement that strategy. Try not to cover your windows in netting or drapes, aim to keep the windows clear and uncluttered, allowing an unrestricted view outside, and allowing the spread of natural light into the room to be as efficient as possible.
Use mirrors, not only to reflect natural window light, but direct the eye to one main area in the room, IE if you have multiple mirrors, have them all defecting the same location / feature.
Colour palette. Use natural and light shades of a colour palette, using shades of colour to introduce depth and space within the room. By unifying colours you can make it easier on the eye, and by using gradations of shades on opposing walls you can use a lighter shade to show distance, or use a darker shade to shorten distance,
Use spotlights and and lighting techniques to highlight features or hidden darker spaces, but make sure you don't add too much light everywhere as this will flatten it all out again - the trick is to still allow for shadows and depth.
Paintings. You could think that having nothing on the walls would be better, but actually it is recommended that you have one larger paining on the wall rather than either nothing, or multiple smaller frames - it's all about creating features, rather than clutter.
Reduce clutter. It's an obvious one, but one of the easiest to implement. Removing unnecessary clutter can really open out the rooms and areas. Floor to ceiling shelving can add height to a room, and allows you to fill the space more effectively. We would suggest that you leave one or two small shelves with limited items on, as this helps to create 'white space' in a sea of busy-ness - which can really add to a sense of de-clutter ironically.
Furnishings - Don't push your furniture up against the wall, this seems counter intuitive, but by allowing some breathing space behind large items such as sofa's and chairs, it really can make the space seem larger, it's not all about the central 'walking' space. Often it is suggested that if you can, choose furniture and large items that are in the same colour range and tonal range as your walls, which allows them to disappear, rather than stick out dramatically. Often using a glass table is a good use space, adding something functional in without it being an eyesore or cluttering up the open floor space. Buying furniture that stands on legs is a good way of reducing the appearance of blocky heavy furniture, as it creates a lifting feel of space. It's a small touch but this can often work out well. You could use multiple rugs to create smaller spaces in a much larger one.
Hopefully there are some tips here that can really help you create a feeling of more space within your home and living spaces. Let us know if you have some more tips or any feedback to the points above!