How to resize 400 videos on a blog?

On one of my blogs I want to change the theme, but to make some of my posts look correct the embedded youtube videos need to be smaller. I already have nearly 400 posts that have videos embedded in them, so doing this by hand is not going to happen.

All my future posts are no problem, its a simple setting, but does anyone know of a way to re-size embedded videos after you have them posted, without doing it by hand? I'm looking for hopefully a plugin solution, or if that doesn't turn out a clue as to where to look in the CSS tags to change the size. I know how to do this for pictures in CSS, I just can't figure out how to do it for videos.

Thanks

Do you have the embed code in your blog posts? If the vids are a predictable size, use a PHP script to edit the blog posts in the mysql database directly - regex the old vid dimensions and change them to your new desired size.

Autumn,

Thanks for the idea, you seem to often have the right answers; unfortunately that is beyond my current skills. My experience with PHP up to this point involves grabbing the code I need from one place and experimenting with placing it where I want it to go on my pages until it works.

So, although I understand the theory of what you're advising, I wouldn't know where to being in actually implementing it, but, if all else fails, I will not be beyond trying to find the resources and information I need to do it. I was just hoping there would be an easier answer.