I have added many categories and sub categories in wordpress blog. When I write a new post and want to select related categories at the time I have to scroll down so much to select the related categories. To out come of this problem we can increase the textarea size of the categories box from wordpress admin stylesheet file [wp-admin.css]. Here is the code:
margin: 0 5px 0 120px;
padding: .5em .9em;
Now, we can increase/descrease the size of textarea from height section..it will reflect in categories tab.