Hanging Pictures and what size to use in what space

I recently had questions on hanging pictures. I look at the space between ceiling and the sofa, or table whatever you are hanging the picture above, and I center the picture between the two. Now, people have a tendency to use too small of pictures in rooms, look @ the space I mentioned that you have room for, the bigger the picture, the bigger the area looks, or, hang lots of pictures, even throw in a mirror in the collection, to make a unit, remembering , when hanging multiple pictures, hang them 3 finger widths apart, when a person decides to hang pictures to far apart, it no longer looks like a unit, the eye travels from one thing then to another, therefore not looking like a unit.. Another thing to remember, stand back and take a look at the space you are trying to fill. Is it more wide than tall, or more tall than wide, use wide pictures in wide areas, don't put tall pictures in wide areas.