You can download a simple small CSS file from GitHub using this link:
https://github.com/marcvannieuwenhuijzen/BootstrapXL
If you add this CSS file to the HTML after the Bootstrap CSS file you will be able to use col-xl-{size}, col-xl-push, hidden-xl, etc. The media-query breakpoint is 1600px;
Update
The alpha release for Bootstrap 4 is available now with native support for extra large screens.
http://blog.getbootstrap.com/2015/08/19/bootstrap-4-alpha/ but it’s breakpoint for extra large is still 1200px.
Update 2 (july 2017)
Stop using bootstrap and start using standard CSS grids, TODAY. You can find an introduction here.