If I set the framebuffer with xrandr like so:
xrandr -fb 800x600

And try to run Warsow in a screen resolution larger then that, say 1024x768 I get this:

X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  130 (XFree86-VidModeExtension)
  Minor opcode of failed request:  10 (XF86VidModeSwitchToMode)
  Value in failed request:  0x220078e
  Serial number of failed request:  7839
  Current serial number in output stream:  7846

If set the framebuffer to 1024x768 I can now run it.

I tracked this down into the xf86SwitchMode code (xf86Cursor.c:216) and it checks against the size and just return BadValue should it not fit. I guess there could be is some magical way to work around this in the driver.

Cheers, Jakob.