The overall size of the form elements is usually controlled by the CSS in your current WordPress theme.
Use Firebug or Chrome Dev Tools to first understand and then change the CSS used for your CF7 form elements.
Also good idea to target CSS classes & ids used within your CF7 forms only, so that your CSS changes don't inadvertantly effect other elements on your website.
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.