Here is a guide to setup War3 in Widescreen for Windows:
1) Make sure the resolution you want to run is supported by your graphics drivers.
If you can select a resolution in your Operating System, then it is supported. If the resolution doesn't exist, read our guide on how to configure custom resolutions in windows.
2) Backup your registry, in case something goes wrong.
START > Run > regedit > file > export
3) Change Warcraft 3's resolution manually.
START > run > regedit
While in the Windows Registry Editor (regedit), locate:
HKEY_CURRENT_USER > Software > Blizzard Entertainment > Warcraft III > Video
4) Make below change in decimal format:
(Right click > Change > select "decimal" > fill in new value)
resheight = 1600
reswidth = 900
Replace width & height with whatever resolution you wanna play in, examples:
852x480 16:9 Widescreen
1280x720 16:9 Widescreen
1365x768 16:9 Widescreen
1440x900 16:10 Widescreen
1680x1050 16:10 Widescreen
1920x1200 16:10 Widescreen
320x240 4:3 Aspect ratio
2048x1535 4:3 Aspect ratio