First, we need to launch RegEdit, and edit the HKEY_CURRENT_USER\Software\Blizzard Entertainment\Warcraft III\Video key.

There are two variables that you must change: resheight and reswidth. Make sure you change the default hex value to decimal value.

Example for widescreen with 1920x1200 resolution:

reswidth = 1920
resheight = 1200

You may have to restart Warcraft 3 for the change to take effect. That’s all there is to it! Have fun playing Warcraft 3 with your monitor’s native resolution.