Baseline (Widescreen) Solution & Issues
Use these command line arguments with the game's executable:
xres=[xres] yres=[yres] as follows.
Game doesn't support widescreen resolutions through Options. To use such resolution you have to add a command line parameter:
"C:\Program Files\Microsoft Games\Age of Mythology\aom.exe" xres=1920 yres=1200
Change the resolution to your liking.
1 ) Set the game to whatever res you want, and click OK. It'll go to that res, stretched to fill the screen.
2 ) Then click on the 'play in window' button in the options on the right. It'll fill the screen. Or it did for me.
Ultra-Widescreen (21:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
Same method, add xres= yres= to your shortcut's Target field. (The SGU entry only forces windowed mode and doesn't even list the Titans expansion, so don't bother.)
Hor+ gameplay, stretched UI except the minimap.
Main menu 2D graphics is stretched, however text labels are OK because they are written with system fonts. Also the background in main menu is OK because it is using game engine. In-game view is Hor+. In-game 2D UI elements are stretched and don't scale with resolution. The text is however scaling with resolutions. The higher the resolution, the smaller the text.
In-game 2D HUD elements are stretched except the text labels. Widescreen cut-scenes (using game engine) are Hor+ and Ver+ against normal.
4k UHD Specific Solution & Issues
Stereoscopic-3D Solution & Issues