Say we are trying to modify the height of the categories meta box of www.glassescanada.com, edit functions.php, which is under:
https://www.glassescanada.com/wp-includes/, add the following code at the end of functions.php
function change_cat_meta_postbox_css(){
?>
<style type="text/css">
.wp-tab-panel, .categorydiv div.tabs-panel,
.customlinkdiv div.tabs-panel,
.posttypediv div.tabs-panel,
.taxonomydiv div.tabs-panel {
min-height: 42px;
max-height: 500px;/* change this to own wishes */
overflow: auto;
padding: 0 0.9em;
border: solid 1px #dfdfdf;
background-color: #fdfdfd;
}
</style><?php
}
add_action('admin_head', 'change_cat_meta_postbox_css');