Blizzard fixed this in Patch 1.25b: Widescreen (stretched) resolutions should now be available in the Video options.
Blizzard Entertainment forgot for 7 years to patch Warcraft III - Reign of Chaos and Warcraft III - The Frozen Throne to list all supported resolutions in the options pane when running on Windows (all versions). Instead, when running on OSX, all resolutions are available (including wide screen). That's just not fair.
The solution is easy for some, complicated for others: edit "reswidth" and "resheight" in the registry at [HKEY_CURRENT_USER\Software\Blizzard Entertainment\Warcraft III\Video].
Search Google and you will find a lot of apps/utilities to do it for you, but some of them require you to install some version of .net platform (Foole's WC3 Resolution Changer), while others incorrectly list the available display modes (or not at all).
There is however an app that will detect the correct resolutions (w3changer), but it has an ugly 16x16 icon (despite its pretty interface), no game launch button and no current resolution detection.
You will also find tutorials (for example www.overclock.net or www.techsupportforum.com or widescreengamingforum.com) that expect you to follow the lengthly steps of editing the registry every time the game resets the resolution back to 640x480 (after making any changes in the options section).
Installing and using the app
- Extract the archive contents;
- Find the directory where Warcraft 3 is installed and move "War3ScreenResolution.exe" in there;
- Right click "War3ScreenResolution.exe" and choose "Send to -> Desktop (create shortcut)";
- Rename the shortcut on Desktop to your liking, for example to "Warcraft III";
- Now everytime you want to play, just double click the shortcut, and choose between "Warcraft III - Reign of Chaos" and "Warcraft III - The Frozen Throne".
Standard widescreen resolutions
16:9 aspect ratio 1024x576, 1152x648, 1280x720, 1600x900, 1920x1080 and 2560x1440
16:10 aspect ratio 1440x900, 1680x1050, 1920x1200, 2560x1600
And yes, these work with DoTA Allstars, and all the other custom maps in Warcraft III.
To check if you are running at the resolution you want (should be the one the program shows you as default), just look closely at the 3D renderings during play. Not at the menus, not in the options/video pane, and not at the game UI during play. Just the 3D world.