What exactly do you mean by flexible?
Currently you have a specific width set for the ad. If you remove the width then Google will select whatever width ad will fit the container. You need to make sure that the container has a width defined if you do that (such as a % of the width of its container.)
The other thing is that if the viewport width becomes too narrow to fit both the ad and the content next to it you need to have the CSS arranged so that one automatically drops below the other.