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.