In case you cannot use the launcher settings option to change the game client resolution please follow this guide.
To change resolutions manually go to game client and open LauncherOption.if file with notepad.
//0: 800x600 (4:3)
//1: 1024x768 (4:3)
//2: 1152x900 (4:3)
//3: 1280x720 (16:9)
//4: 1280x800 (16:9)
//6: 1440x900 (16:10)
WindowMode:1 ON - 0 OFF
Example of a 800x600 resolution launcheroption.if file.
Example of a 1024x768 resolution launcheroption.if file.