Being able to change the size of a room without knocking down or putting up walls is possible. All you need to do is add some paint and a few accessories. I would like to share with you a few different ways you can do just that.
To shorten a long room
Painting a darker accent wall gives the eyes something to focus on as you enter
and it takes the focus off the length of the room.
To enlarge a small room
Keep the area a light color throughout. It doesn't have to be a completely white room but the lighter colors keep the room open. Also the more light the better. If you do not have a lot of natural light, you may want to add some decorative lighting.
To elongate a narrow hall
Painting the wall or door at the end of a narrow hall and the ceiling with an accent color will visually shorten the hallway and give the illusion of width. Another little trick that will visually widen a narrow hallway is to paint doors leading to other rooms a darker shade than the walls around them. I have also seen applying large mirrors opposite each other and gives the illusion of a large space.
Bring the paint from the ceiling down on to the wall and using this technique helps bring the ceiling down and give the impression of shorter looking walls. Also adding a border or wainscoting adds a little extra boost to the room painting different colors or keeping all the same.