James Padolsey's and ahaxe's jquery scripts are two very good instances on autogrowing form fields.
Likewise, autosize can be accomplished in a number of ways using Javascript.
But, just wondering, is there anyway to do any kind of autosize or autogrow with CSS only? I don't know CSS deep enough, so you know, with all those pseudo-classes and CSS3, I'd guess this might be possible.
Or maybe the only option would be using either <input size> or style="width: em;"...