Wrong- there is no default screen resolution for an operating system. It's your hardware that has a default screen resolution
If you have Mint or any Linux installed you can open a terminal and issue the command xvidtune, click cancel in the box that pops up. Now you have the vertical and horizontal referesh rates for your monitor
Those values are to be used in xorg.conf for you to be able to use the resolutin you want.
If this does not help you (maybe to geekish
) ask again and tell us what hardware you use, specifically video card and monitor