A long horizontal shape would accentuate width, while a tall vertical shape would call attention to height.
An angular shape such as a square or rectangle, gives an orderly, restrained look, while a rounded or curved frame could be softer or even whimsical.
Size can influence the impact you want to create. The selection of size should also be based on whether you want the mirror to be a focal point, an accent, or merely act as the background.
If you want your mirror to be focal point in your room, make sure it is large enough to stand out, but also choose a frame that makes it even more noticeable.
If you want your mirror to become part of the background, you should choose one that is extra large so that its function then becomes to make your space appear larger.
Smaller mirrors can act as accents, catching light here, reflecting an interesting object there. They can also be used in groups. If you do plan to use smaller mirrors in groups, remember that together are acting to form a much larger shape and could act as a focal point in the room.
Your mirrors should reflect the style, you already have in your room. While the shape and size of a mirror matter, the frame plays an even greater role in determining style.
An ornate, carved or gilt frame can be used in a more traditional setting.
A more simple frame treatment would be suitable for a casual or modern interior.
Whimsical frame treatments and distressed looks work well for a country look.
Source : http://furniture.about.com/