A resonable default value for the screen resolution could be retreived by using this code <http://blogs.msdn.com/oldnewthing/archive/2004/07/14/182971.aspx>:

int GetScreenDPI()
{
  HDC hdcScreen = GetDC(NULL);
  int iDPI = -1; // assume failure
  if (hdcScreen) {
    iDPI = GetDeviceCaps(hdcScreen, LOGPIXELSX);
    ReleaseDC(NULL, hdcScreen);
  }
  return iDPI;
}

The routine could be injected into gtksettings.c - similiar to the code for the X11 backend.