Running selenium tests via Team City as a windows service has a resolution of 1024x768. I tryed adding/changing DefaultSettings.XResolution and YResolution in the registry with no luck. What did work was running TC agent in the console as a logged in user.
Use these instructions to setup a user to automatically log in when your box starts up
http://www.win2008workstation.com/configure-auto-logon/
Add shortcut to the startup forlder on start menu: C:\Users\YOUUSER\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup
that points to your team city agent's start.bat with 'start' parameter