Responsive select box

[ Moderator note: title fixed. That’s not how you “resolve” topics, use the drop down menu on the right (as I’ve just done). ]

I’m using the responsive menu on http://praktijkvoorcgt-emdr.nl.greenhost.nl/
It works nice but the select box remains it’s full width on a small device. How can I make it responsive?

http://wordpress.org/extend/plugins/responsive-select-menu/