change menu container width

as editing the stylesheets of the theme directly is not recommended, you might need to add some adaptive CSS via a ‘custom CSS’ plugin or into style.css of a child theme.
https://ww.wp.xz.cn/plugins/search.php?q=custom+css
http://codex.ww.wp.xz.cn/Child_Themes

what exactly you need to add depends on your currently used theme.

what theme are you working with?

http://codex.ww.wp.xz.cn/Forum_Welcome#Include_as_much_information_as_possible

can ou provide a live link to your site to illustrate the issue?