[Greasemonkey] changing style with javascript

[Vectorspace]

I have a div with an undefined height - so the div will assume whatever 
height is required for its content.

I use this to give it a height of 0:
document.getElementById("div_id").style.height="0"

I now want to undo that change, so the div will again be whatever size 
is required by its content. How do I do that? I tried style.height="" 
but that didn't work.