Set height of the categories meta box in post dashboard

If I am correct the style.css (your theme style.css) is not loaded in the admin area so creating a function solves this.

Side note: The code you where looking for I found in wp-admin/css/edit.css at line 960

Below you find the function,hopefully this is what you where looking for.
Make backup from functions.php (you know the drill)

Add following function to 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');

Take a look here for some more info.

Just as a reminder, to change layout in the admin area(back-end) you need to create a function and the correct hook to achieve your goal.

side note 2: It probably changes height for all meta postboxes but find the right div and add it to the function should also solve that, cheers.