Auto-Resize Video

Hi! Do you know of a plugin or script that can automatically re-size a video (youtube, dailymotion, etc.) to fit a specified div size?

I have an existing wordpress site and I need to change into another theme. Unfortunately, most of my videos do not fit the post page because it is smaller than the size of my videos. I’d rather not edit each and every post to fix it if there’s a plugin or script available.

I tried searching for one but could not find any. Your help is appreciated. Thanks!