You can set options like below:
[text* your-name “Your Name” 100/200]
This means “size is 100, maxlength is 200″. And apparently, if there is CSS which defines width of textareas (e.g. textarea {width: xxxx}), the CSS setting override Contact Form 7’s setting (cols attribute of <textarea> element).
However, all I have in my CSS is
body, select, input, textarea {
font: 100% Arial, Verdana, Sans-Serif;
}