Your current theme appears to have no style settings for the width of input boxes. Your current widths would be based on default browser settings.
To change the style of your CF7 forms you would need to edit the CSS style sheets used by your WordPress theme.
See Styling Contact Form for a general explanation of styling CF7 forms using CSS. If you are not familiar with CSS, this page also includes some links to where you can learn CSS.
Use Firebug or Chrome Dev Tools to find and target individual css classes & ids within your CF7 forms so that your css changes don't inadvertantly effect other elements on your website.
Also looks like you have set the maximum character limit in your message box at 2000. You can use Firebug or Chrome Dev Tools to see what is going on here as well.