Think that call to the CSS file is OK, 'tis one weird filename though....
Far as I know you can set a fixed width only. You could do it by inline CSS, per fieldset, if needs be:
<fieldset style="width:100px;">
Test in different browsers though, the IE box model means a slightly different interpretation of the width value (allow enough for it not to matter, or look at box model hacks or IE's conditional processing).