Accentuate the vertical
Our brains associate height with a sense of spaciousness and openness. By accentuating the vertical elements (tall furniture, floor-to-ceiling curtains, vertical stripes on walls etc.) in a space, the illusion of height is created by giving the impression of a higher ceiling.
Use see-through furnitures
Gone are the days of normal, boring furniture. Solid and opaque furniture are visually obstructive due to the visual space they take up. Transparent furniture, however, allows light to pass through, creating a sense of openness and space in a room. See-through furniture has clean lines and minimal visual mass, preventing it from visually overpowering a room.
Photo credit: https://www.coasterfurniture.com/acrylic-furniture
Brighten up the space
As compared to dark spaces, bright spaces tend to reflect more light. This creates a visual effect that expands the perceived boundaries of the room. When shadows are minimized, a space becomes more uniformed in terms of illumination. This results in a sense of openness. We all want to feel more comfortable and relaxed in a room and by using bright spaces to create a positive and uplifting ambience, it will give off a cozy atmosphere.