I've tried helping others with AdSense resizing problems before, and the problem is that the ads get output inside an iFrame. When you pick a particular ad format, you pick a specific size, and the iFrame code comes with the width & height attributes set. While you can override the size attributes for the iFrame by adding width & height CSS properties, you can't touch the contents of the iFrame with CSS, since it reflects the contents of another site, so resizing the iFrame, e.g., making it smaller for a narrower view port, will generally cut off parts of the ad.
This problem is more apparent in AdSense ads which have a wide, horizontal format which are intended to fit in the main content area of a page, rather than the vertical, tall thin ads which are intended to fit inside a sidebar. So if you do have the wider ads, I would consider switching to the vertical ones and putting them in a sidebar, or in a narrow widget area of the footer.