You need to enclose your contact form in <div></div> tags on the contact form page. Easiest to do it there. Unless you are using the form in several places with varying widths.
<div style="width: 400px; margin: 0px auto">
(enter your form here)
</div>The width in the above can be any size but you have to make sure it fits in your page or post width. If you don't know, then use percentages and it will be more fluid - width: 80%. This is all simple coding... have fun with it!