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 size of the standard HTML size attribute which may or may not control overall element size.
If you want to control the width of fields, you can do it by editing the CSS stylesheet in your theme. For details, see Styling Contact Form.
There is a link at the bottom of the page to a comprehensive and detailed article on Styling Contact Form 7 Forms. The article shows people, with suitable HTML & CSS skills, how to change the appearance of their Contact Form 7 Forms to meet their particular requirements.