if (screen.height <= 719) //if 800x600 
document.getElementById('txtscroll').style.height = 9.0 + "cm"; 
else if (screen.height >= 720 && screen.height <= 767) 
document.getElementById('txtscroll').style.height = 10.7 + "cm"; 
else if (screen.height >= 768 && screen.height <= 863) 
document.getElementById('txtscroll').style.height = 13.0 + "cm"; 
else if (screen.height >= 864 && screen.height <= 959) 
document.getElementById('txtscroll').style.height = 13.5 + "cm"; 
else if (screen.height >= 960 && screen.height <= 1023) 
document.getElementById('txtscroll').style.height = 15.0 + "cm"; 
else if (screen.height >= 1024) 
document.getElementById('txtscroll').style.height = 17.0 + "cm"; 
else
document.getElementById('txtscroll').style.height = 9.0 + "cm";