If you have a popular blog, or a blog you haven’t check on in a while that is now filled with thousand of comments, you will spend half a day deleting comments 20 at a time.
There are all sorts of DB hacks you can do, but this is the simplest way to display more than 20 comments in moderation.
Chances are they’re all spam, so it’s not worth looking through them all (if the blog is not actively moderated).
1. Find the file /wp-admin/edit-comments.php and dowload it to your computer.
2. Find line 189 (as of WordPress 2.8.4) that says $comments_per_page = 20;
3. Change the number 20 to whatever number you want. 100, 200, 500 whatever. Edit: Actually, scratch 500… that might be too ambitious, and you will get a request URL error. 250 should be the max
4. Re-upload file.
That’s it. Easiest way to do it.