Taken from
http://www.zeeronsolutions.com/how-to-make-contact-form-7-plugin-fields-responsive-full-width/

/*-----------------------------
Making WordPress Contact Form 7 Plugin form responsive and full width with CSS customization
Add this code snippet to your WordPress theme's style.css file.
-------------------------------*/
div.wpcf7 .wpcf7-form-control {
max-width: 95%;
}

/* Make Contact Form 7 Select Dropdown field responsive & full width */
.wpcf7 select {
background-color: #fff;
border: 1px solid #ccc;
border-radius: 3px;
margin: 0;
padding: 4px;
text-indent: 0px;
width: 100%;
z-index: 100;
}
/* Make Contact Form 7 text field full width & responsive */
.wpcf7-text {
width: 100%;
}

/* Some additional CSS customization to the whole Contact Form 7 form */
div.wpcf7 {
background-color: #f5f5f5; /* Adds background color */
border: 2px solid #eee; /* Adds a form border */
padding:20px 10px 10px 10px;
-webkit-border-radius: 4px; /* Making form border rounded */
-moz-border-radius: 4px;
border-radius: 4px;
}