[jQuery] Set Div Width

I'm setting the width of a DIV using:
...).children('div').attr( {width: ('80px'} );
It seems to have no effect (I have margin:0 auto; set in CSS).
The rendered (view/generated source) shows:
<div width="80">
which isn't what I had expected. I know that:
<div style="width:80px">
will work fine.
Is there a better way to set the width of a block element?
Sam