Before you start editing files I mention, save a copy of the original file BEFORE you start editing it. WHEN you break your system, boot to runlevel 3 (text, multiuser), put the original files back and you should be able to use your computer again.

In the file /etc/X11/XF86Config, search on "Screen" and you will see under Subsection "Display" the WxH values that you mentioned. If you want to add a new WxH, be sure that your video driver and monitor can both handle it. Remember, the limitations might be hardware, not just software. You will notice that there are three sections: Standard, Accelerated, Fallback. You can change them all, but I would leave Fallback alone.