By default it can go up to 1920 x 1080 pixels. If that's not big enough for you, you can always go in your config.xml file; that can be found in the following directory: "C:\AeriaGames\GunZ". You can edit your resolution by right clicking this file, opening it as a notepad, and replacing the values in these tags "<WIDTH>1680</WIDTH> | <HEIGHT>1050</HEIGHT>" to whatever you want as long as it maintains a 4:3 ratio and does not go below 400 x 300. It is possible that you get an error dialog when starting GunZ, it simply means you applied an incorrect resolution.