if (window.innerWidth) { 
  breite = window.innerWidth;
  }
else {
  breite = document.body.clientWidth;
}

if (breite <=700) {
  stylesheet = '/fileadmin/css/size_small.css';
}
else if (breite <= 800) {
  stylesheet = '/fileadmin/css/size800.css';
}
else if (breite >= 1100) {
  stylesheet = '/fileadmin/css/size1100.css';
}
else {
  stylesheet = '/fileadmin/css/size.css';
}
document.writeln('<style type="text/css" media="screen, projection"><!-- @import url(' + stylesheet + '); --></style>');