Check out this free module on the Apps Store:

https://www.odoo.com/apps/modules/8.0/web_menu_hide_8.0/

It introduces a button on the left of the main menu bar (next to the 'Messaging' menu), which you can click to toggle the left menu bar on/ off.

You might need to edit the JS file 1st line. Make sure it is:

openerp.web_menu_hide_8 {

in the file openerp/addons/web/static/src/css/base.css

.openerp .oe_leftbar {
  display: none;
  width: 220px; 
  background: #f0eeee;
  border-right: 1px solid #afafb6;
  text-shadow: none;
}

Change the width above to a smaller value.

Also remove the width from .oe_application

.openerp .oe_application {
  width: 100%;
  height: 100%;
}

you can use firefox or chrome to inspect the element and make changes to the css style scripts beside and see the effects before deciding if you want to change it on code.