Dynamically Resize – Mega Video etc.

Hi i was just trying to figure out a way of resizing a video dynamically without having the change the dimention and so it fits within the post box and can be resized automatically dependant on resoultion

I can do this by manually chanding the width dimention to 100% within the actual embed code, but i cant seem to find out how to hard code it into the CSS or PHP so that it says: Oh hai your a video your width should be 100% not x ammount of pixels.

I found the

.post embed, object {
width: 100%
height: 100%

but had no luck getting this working. Thanks a lot.