No matter what kind of room you have, irrespective of slanted or straight walls, light colors can always make a room look bigger. If you want just the ceiling to look higher, paint it in some light and cool color. The same goes for the walls as well. To give the room an illusion of height, vertical lines must be used. It is not necessary that you use them on the walls, furniture with vertical shapes like the canopy bed or a long cupboard can be used.