Proper lighting has a significant impact on the overall mood and design of a room. A light fixture can change the whole mood of an entire room from a romantic setting to a work area.
If you plan to re-purpose a room, you should start with the lighting. The goal should be the atmosphere you're trying to create. If you're undecided about a rooms function, you can combine different fixtures for added flexibility. Thus, a bedroom can be an office and vice-versa.
You should first figure out what the room is going to be used for and where the lighting fixtures will be installed. Most rooms can serve multiple functions, so choose focused lighting, like sconces for a desk area where you will be working and maybe a chandelier if you're going to receive guests. A chandelier will spread light over the whole space of the room.
For kitchens and bathrooms, you should choose multiple lighting sources. The kitchen requires a bright overhead lighting scheme since it's the place where the family members gather and eat. Also, in the counters area, there should be some sort of focused lighting since this is where the food is prepared and where the dishes are washed after eating.
For bathrooms, the diameter of the lighting fixtures should be about a third of the vanity width. Also, lights placed around the mirror ensure a proper illumination of your face when looking in the mirror.
For the bedroom area, you should opt for lights that create a sensation of relaxation. In the bed area, tabletop lamps will create a bit of focused lighting for reading. As far as the ceiling goes, a good central light which can spread diffused light is perfect, since it will illuminate the room in an equal manner. Sconces on either side of the bed will provide the perfect lighting conditions for reading in the evening while in bed.
Ceiling mounts can provide proper lighting in areas with high foot traffic like hallways, foyers and stairways.
Before deciding on your lighting fixtures, you should measure up a room, and convert the sum of the width and length of a room from feet to inches. This will be the maximum diameter of the central lighting fixture you need. For example, if a room is 10 by 12 feet, then the sum would be 22 feet, thus you'll need a chandelier with a maximum diameter of 22 inches.