the widget widths are set with CSS, but usually the div#sidebar width is set and that controls the widget width, for example here on buddypress.org:
`
div#sidebar {
width: 224px;
}
`
thx,
Is that in a particular .php file?
Is there somewhere I should go to look for it?
It’s not a php file . CSS styles are contained in files ending .css either styles.css in the theme root or files in the folder /_inc/css – however don’t edit core files if you can avoid it copy required files to a child theme folder; a guide on doing so can be found in the BP codex.