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
C:\TeamCity\buildAgent\bin\agent.bat start
On startup a console will launch that is the TC agent and it will connect to your server and work. I still have the TC agent installed as a windows service, but disabled an set to manual startup.
My TC server is in a VirtualBox VM and I setup 2 networks cards. 1 is the normal NAT card, and one is 'Host-oonly Adapter'
The host only one gets me a 192.1686.x.x address that I can hit from my host machine. Now I Just have a new tab up for VM's TC install, and I can configure/kick off builds with the vm minimized!