'Cause you haven't tried.
I tried in rockstarkeynotespeakers.com and it works:
.wpcf7-form-control {
border: 1px solid #BBBBBB;
border-radius: 5px;
margin-top: 0.5em;
margin-bottom: 0.5em;
padding: 0.6em 0.3em;
min-width: 100%;
}
If you want you can remove min-width and adjust with padding.