Veneers are a great option for correcting colour, shape, size, and position of teeth - big or small. Orthodontics is a great option for correcting position of the teeth. A combination of both treatments may also be an option for you, but it is hard to tell from the photo provided.
One of the issues that can come up with treating only 6 teeth is that the untreated teeth will not match those new Veneers. The effect of this is that the 6 new veneers will possibly look bigger, whiter not naturally part of your smile.
If you are serious about getting the best aesthetic result possible with Veneers you will want to consider including all the teeth you show when you smile. Especially if you are trying to make changes with Colour and size.
You can, in many cases, reshape your teeth so they don't look so "big" with Veneers but again you have to take into account your whole smile, not just the "Social Six" - the ones YOU see in the mirror. In most social situations people are looking or talking to you from a slight angle and they WILL see your side teeth. And we humans are very good at picking up on discrepancies in colours and shapes. If you want the best result possible, don't forget these teeth.