Resizing Css Div In Javascript

I have a CSS div defined as follows :
<div id="col-2" >

In some circumstances I want to change its width.
I have tried all the following within a javascript function:

document.getElementById("col-2").width= "900";
document.getElementById("col-2").resizeTo(900, 520);
document.getElementById("col-2").width.value= "900";
document.all.col-2.resizeTo(900, 520);
document.layers["col-2"].resizeTo(900, 520);

None of which have worked.

Within the stylesheet I initially had col-2 defined as an ID. Clutching at
straws I changed it to a class but that made no difference.

Can someone point out the right way to do it?