Scale and Proportion
In design terms scale and proportion are closely related. Scale is generally an indication of the size of objects as they relate to each other, to people, and the space they occupy. Proportion is an expression of the comparative relationship between a part or parts and the whole. An enormous dining room table squeezed into a tiny space is out of scale with the size of the room and perhaps out of proportion with the nearby furniture. A child's chair may look out of scale along side the living room sofa but perfectly at home in the child's bedroom along with other objects. That's not to say you cannot introduce an object that is out of scale to make a decorative statement. An overstuffed chair can make a small space seem more intimate, or a diminutive chair in the company of standard sized furnishings can serve as an interesting foil to the eye.
Rooms and their furnishings seem in proper scale when they fit psychologically and physically with the people who use them. They seem in good proportion when they express unity and feel balanced. In a successfully decorated room, the seating relates well to the other furnishings and is sized to accommodate the people who use the room.
The windows and their coverings seem to take up the right amount of wall space and the artwork over the fireplace mantel looks just right whether you view it from a sitting or standing position. Achieving a scale that's appropriate for you and your spaces may take some practice. Your intuition can help give you guidance.