Basically, you need to balance two things: a soundbar that fits within the space you have available, and a soundbar that fits with your TV. Some people have wall-mounted TVs with a soundbar mounted underneath. Others have their TV on a stand and the soundbar just sits in front of it (the height of your stand is quite important if this is you, because nobody wants a tall soundbar that covers the infrared receiver on your TV).
Consider the space you have, but also the size of your TV. A compact soundbar might look silly beneath a massive screen and vice versa – if your 'bar juts out at either end of the screen you might have bought the wrong one for your needs.
The good news is that soundbar manufacturers know what they're doing and have a keen knowledge of the products they're designing for. There are many different-sized soundbars available to suit all the TV screen sizes on the market, so make sure you check the dimensions and match them up.