window.onload = ridimensiona;

function ridimensiona(){
  fl = document.getElementById('cont');
  if(screen.height < 768){
    fl.style.height = '450px';
    fl.style.width = '611px';
  }
  else if(screen.height < 870 && screen.height > 700){
    fl.style.height = '600px';
    fl.style.width = '814px';
  }
  else if(screen.height > 870){
    fl.style.height = '700px';
    fl.style.width = '950px';
  }
}