Hi DC,
It depends on the layout you're using, the mainbody part has a container class which is set with a fixed width value.
For example, if you don't use sidebar, you can open this file:
/templates/ja_mixstore/tpls/blocks/mainbody/no-sidebar.php
Look for this line:
<div id="t3-mainbody" class="container t3-mainbody">
and remove the container class.