It's a commonplace observation that mirrors can extend a room. But the observation is commonplace for a reason. It's true. Putting up a mirror or mirrors creates the compelling illusion of more space, of another whole area just through the wall. So decorating with wall mirrors is a smart choice. When deciding what sort of mirror you want there are several things to consider.
Where should you put the mirror or mirrors? For the mirror to do its work of extending the room, it needs to be in a location where people will naturally look. And it needs to be opposite plenty of light. (That's why mirrors along a hallway rarely work out as well as one might hope.) One obvious location is over the fireplace. But there are plenty of others. Stand in the room where you want to put a mirror and notice where your eyes naturally go. What direction are people most likely to be looking in? That's a good place for a mirror. But be careful! Stand where the mirror will go and look at what it will reflect. If it's going to reflect a TV and DVD player, that may not help you achieve the elegance you're looking for.
Another thing to consider is how large should the mirror or mirrors be? Naturally, this will be determined in large part by how much space you have on the wall and what else you might want to put there. Remember that the space doesn't necessarily have to be filled with one large mirror. Several small mirrors can do the same work and have a delightful effect of bringing that extended room into and out of one's field of vision. Perhaps a large mirror flanked by two smaller ones will provide the look that you want. Perhaps three mirrors arranged in a triangle or four in a square will be what you're after.
What shape should your mirror be? That's a decision that calls on you to have a personal "style." You can get basic rectangular or square mirrors. Or you can get mirrors that are circular or oval. If the one seems too angular and the other too curvy, you can get basically rectangular mirrors with curved tops and bottoms. Or perhaps you'd like one with just a curved top, creating the illusion of an archway into another world.
The border of the mirror can be made of all sorts of different materials. If you want a wood border, you can get cherry, mahogany, oak, etc. If you want a metal border, you can get anything from polished brass to wrought iron to brushed nickel to chrome. You may even want to dispense with the border altogether and just put up a plain mirror. Perhaps the edges can be beveled or otherwise distinguished from the rest of the mirror so the mirror contains its own border. A wicker border is also a possibility.
Mirrors can be as ornate or as plain as you want them. A simple, thin chrome border can provide all the frame you need, allowing the mirror to do its work. Or you can get a fancy wrought iron border with filigree and flowers and leaves. A metal latticework border can also look quite stunning. Wooden borders can be plain or they can have all sorts of different carved material, from flowers and leaves to musical instruments to animals and people. You may even want a mirror that has a pattern across the front, like a wrought iron gate. This won't work if you want to use the mirror regularly to brush your hair and apply makeup. But as a romantic effect in the living room or entryway, it may be perfect.