kouliga Increasing global layout width would require you to make other modules and elements in-line with the layout width henceforth, requiring several changes in order to get everything fine in appearence as you have already come quite far with your site
So, Would suggest to either go for this option for adding this to custom.css file as this would solely affect the container width for main-menu along with other css rule-set:
#t3-mainnav .container{width: 1200px !important;}
.navbar-nav{margin-left: 3em !important;}
and adjust values accordingly although, this is still a suggested workaround for which you may require to check rigorously across varied screen sizes and resolutions if it adapts fine and doesnt require further css tweaking
The most viable solution would be to disable one of main menu item in order to accomodate off-canvas toggle with ease