While I've been looking around on peoples member pages, I've been seeing people edit their comment boxes so it's a certain size. They've done this by creating a CSS rule for the id blog_comments_box. Now this is fine, but the problem with it is when you scroll, your blog comments title goes with the scrolling. So a tip for you would to be instead of creating a CSS rule for blog_comments_box, create a CSS rule for site_comments_div. For example:
You will also notice in this example that there's a rule for "max-height". Now the reason I have done "max-height" is because if your blog doesn't have many comments on it, then it might not go up to 400 pixels. So the max-height will make it so it can be smaller than 400 pixels, but not higher.
Hope this helps.
I have done this for the guilds comment box if you would like to see how it works.