There are a few things that can be done.

Mirrors are used to give it a false sense of being bigger than what it really is. A well placed mirror can be misleading and seem bigger.

Painting or wall papering the ceilings giving the effect the room is taller than it actually is.

Painting the walls a light color as it reflects the light making it seem more airy. Dark colors make it more cozy.

Pull the furniture away from the wall as it gives the illusion of spaciousness. This gives a more open feel.

The use of stripes in a carpet will help make the room look longer. Your eyes are drawn to different things and creates illusions.

Certain decorations like large vases but not too many. The smaller the item the less impact.

Leave the curtains open allowing more light and it also draws the persons eye fooling the brain how small the room really is.

Obviously if you have a box room there is nothing you can really do except have hidden storage in furniture etc. The less clutter the better.

These are all tricks with light, color and reflection that the professionals use when selling a house or an apartment.