Maybe a tiny little late but my working solution looks like this:
[[!FormIt?
&hooks=`recaptcha,email,redirect`
&recaptchaTheme=`clean`
&recaptchaWidth=`42`
&recaptchaHeight=`23`
...
]]
The resulting HTML code looks like this:
<script type="text/javascript">var RecaptchaOptions = {"theme":"clean","width":"42","height":"23","lang":"de"};</script>
As you can see &recaptchaWidth -> width, &recaptchaHeight -> height.
But width and height actually do not have an influence on the the look of the recaptcha.
[ed. note: mathis last edited this post 11 years ago.]