Part of the fun in being a designer is thinking outside the box and coming up with new and creative ideas.... however, there are some rules of thumb that it just makes sense to follow, well, because they work. Here is a little designer-cheat-sheet that may come in handy during your next project.
1. 1 yard = 3 feet (you wouldn't believe how much time I spend converting units of measurement)
2. When hanging a singe piece of artwork, the center of the piece should be roughly 56"-60" from the floor, so that it will be at eye level.
3. When hanging art above a piece of furniture (a chair, table, mantle, sofa etc) leave about 6-8" of space between the furniture and the bottom of the artwork.
4. A ceiling light fixture that does not hang over a table (basically one that will be walked under) needs to hang at least 7 feet above the finished floor.
5. Same goes for ceiling fans, these should be hung so that the bottom of the fan is at least 7 feet above the floor.
6. the bottom of a chandelier or other over-table fixture should hang about 30"-36" above the top of the table for regular 8' ceilings. Add about 3" of height for each additional foot of ceiling height.
7. Wall sconces should be hung about 60"-72" from the floor unless they are being used to light a specific ares (i.e. sconces as bedside lamps or for hanging a sconce over a piece of artwork)
8. Twin Bed Size: 29"w x 75"L
9. Double/Full Bed: 54"w x 75"L
10. Queen Bed: 60"w x 80"L
11. King Bed: 76"w x 80"L
12. Draperies should withe brush the floor (barely skim the floor) or puddle on the floor (about 2"-3" of extra fabric flows on the floor). Draperies should never stop short of the floor.
13. Drapery rods should always extend 3"-4" past window frames, otherwise the draperies will block the window too much and minimize the light.
14. To make your windows look wider, and maximize on the light you let in, extend your rod 6"-8" past the window. That way the full width of your draperies will hang against the edge of the window.
15. To make your windows seem taller hand the rod anywhere from 4"-12" above the top of the window frame. (Only make sure to make the draperies accordingly so that they will still reach the floor.)
16. Drapery Panels should each be 1-1/2 the width of the window in order to be nice and full.
17. When making Roman shades, be sure to measure either the insides of the window frame or the insides of the window trim... but make sure to be consistent. I recommend measuring the inside if the window frame.
18. A rug placed under a table should be at least 2 feet wider than the table on all sides.
19. For most rooms, there should be 10"-20" of floor between the edge of an area rug and the wall.
20. When prepping for a project, make sure to have the correct measurement of the space you are working on prior to shopping or purchasing materials. The worst mistake you can make is to "eyeball" something, because chances are it will not be right.
Of course, these are just guidelines! Half the fun in decorating is breaking the rules and coming up with your own ideas!