1. Eye level means the center of the picture is about 60" from the ground. This will seem low, but trust me, it's right. The average height is about 5'8" (for men and women, combined) so this works out well for just about everyone.
2. Artwork hung over a sofa or other furniture should be about 4-6" (but no more than 8") from the top of the piece. I don't care how high your ceilings are. Art hung over furniture needs to relate to that furniture. For more height, hang a grouping.
3. For groupings, have either all of the tops or bottoms of frames hung at the same height. It's an easy way to create a clean horizon line that's more pleasing to the eye.
4. Walls smaller than 36" should more often than not be left bare to avoid the "crowded" look. Better to group several small pieces together than hang them piecemeal througout a room.