[Greasemonkey] max-width textarea?

[Chris Hayes]

I need to limit the width of a textarea (name="bericht", no ID, no CSS 
class).
I tried to start with max-width.

This works:
unsafeWindow.document.forumpostform.bericht.style.width='300px';

This does not work:
unsafeWindow.document.forumpostform.bericht.style.max-width='300px';

is there a solution for this?