If you are planning to add responsive Google ads then we need to take care of the size of the parent container of the Ads. This is compulsory that size should be in percentage(%). I tried to use this setting and set container's size as 50% in a particular part of the page but google ads was taking very long width. The problem is the layout of the page. Google do not measure site against that particular section of the page but it measures size against the screen's resolution.

For example: if we have structure like this:

<div class="parent&#95;container" style="width:200px">
   <div class="container" style="width:50%>
          space for ad
   </div>
</div>

And our screen size is 1280X600, then width of ad will be 640px not 100px (which is required) So if we need to use responsive ads in such conditions then we have to use " advance responsive ad" instead of "smart sizing"

More information available at:
https://support.google.com/adsense/answer/3213689?hl=en and
https://support.google.com/adsense/answer/3543893#adv