Hi
The Width of the sidebar is defined in the php file .
You can find it in the purity_iiitplsblocksmainbody There u can see a no. of mainbody files , Open a file for example
one-sidebar-left.php Find the code
<!-- SIDEBAR LEFT -->
<div class="t3-sidebar t3-sidebar-left col-xs-12 col-sm-4 col-sm-pull-8 col-md-3 col-md-pull-9 <?php $this->_c($vars['sidebar']) ?>">
Change the size of col-md-3 (span3) to col-md-2 (span2) . Make sure total size would be 12 including content block size .
Here > > > http://prntscr.com/98jlfz
The same way you can change size for other layouts files that u are using .