When I refer to 'art' I mean anything that hangs on the wall for display. It may be paintings, portraits, photos, metallic art, collections (plates for example).
We often hear we should hang our art at 'eye level'. But who's eye level? Eye level when sitting or eye level when standing? Well here are a few pointers I have gathered together to help take the confusion out of it. Generally speaking art should be hung so that the center of the piece is between 57 and 60 in. from the floor. (Most galleries use 60 in. as their benchmark.) However, if you are hanging the piece above a sofa, the bottom of the frame should be between 6-8 inches above it. If hung above a console/table 4-8 inches is good so that the art relates with the vignette below it on the table. If you have high ceilings, you can hang your art slightly higher. Some people hang their art lower in rooms where people will mostly be seated (such as dining rooms) and higher in foyers/hallways where people are mostly standing. Personally, I prefer to stick to the 57 to 60 in. rule except when placed above furniture (as explained above).
Keep large pieces of art on large expanses of wall and smaller pieces on the smaller walls. Putting small pieces on a large wall gives the sense that the pieces are 'floating' away, which is very visually distracting. The exception to this rule is when several similar smaller pieces are grouped together and reads as one large piece. When done correctly this can be a great way to display your favorite photos or collections, etc
Shape.....does it matter? Generally speaking, vertically hung pieces can visually make a low ceiling look higher and a high ceiling lower. Vertical pieces of art and other vertical lines in a room tend to make a room more formal. Lines of a horizontally hung piece emphasizes the width of a room and can visually expand it. It also lends itself to a more casual look.
A couple of more general rules....if hung above a piece of furniture, such as a sofa, the width of the piece should never be wider than the piece of furniture. Some will say the art should not exceed 75% of the width of the furniture below it. Also, when hung above a sofa or similar, the art should take up 2/3 of the wall space between the furniture and the ceiling.