^
While in the Windows Registry Editor (regedit), locate:
HKEY_CURRENT_USER > Software > Blizzard Entertainment > Warcraft III > Video
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