The overall size of the form elements is generally controlled by the CSS in your current WordPress theme rather than the “size” you use in the Form section of the CF7. This refers to the standard HTML size attribute which may or may not control overall element size.
TO change the size (width and height ) of the text area, see Styling Contact Form for a general explanation of styling CF7 forms using CSS.
There is a link at the bottom of the page to a comprehensive and detailed article on Styling Contact Form 7 Forms.
Use Firebug or Chrome Dev Tools to to examine the HTML and CSS of your CF7 form in detail. Once you understand how the HTML and CSS is configured in your CF7 form, you should be able to see the CSS changes you need to change the appearance of the form to suit your requirements.
