Hi,
The width is up to your Joomla Template and has nothing to do with our VM Template (cleanMart)
(you are using Free Gantry Joomla Template, just like our DEMO page. So it is up to Gantry in your website)
The "Gantry Template" uses this CSS file:
/libraries/gantry/css/grid-responsive.css
to define the width size of the elements when the layout is responsive.
(I won't recommend you to touch those values unless you are familiar)
Regards,
Support