Here's something that may work for you.  It's a little hacky, but if you're trying to find a good width for some text, this is the only way besides javascript that I know of.  We're basically forcing the width by not allowing the line to break.  You can put in <br/>s if you need line breaks.
<div style="position:absolute;top:50px;left:50px;background:green;width:0px">
 <div>
  <div>test</div>
  <div style="height:50px; white-space:nowrap">This is normally sized in IE6</div>
 </div>
</div>
On second thought, don't check out the link.  It's old and doesn't work as advertised.
Old answer:
http://snippets.dzone.com/posts/show/216
I believe that non-absolutely positioned DIVs automatically expand to fill their container horizontally.  Since you haven't specified any container size for this div, it expands to fill the whole page.  
I find it odd that Firefox doesn't expand the div... I'm not sure which of them actually has it "right".