i think that it´s dificult because recaptcha plugin uses external css
https://www.gstatic.com/recaptcha/api2/r20160811171912/styles__ltr.css
maybe works if you find in the code the call for the remote css and change the url for a local copy of the css .
then you can search for .rc-anchor-normal in local css code
and change the widht. For example
width: 200px;