The Responsive ad placeholders are a good way to understand where there is potential for your site to earn more on a selected group of readers.
You can set up Responsive ad placeholders to show when readers are accessing your site on larger monitors. These are especially good if you are a tech site or business site where your readers might have access to larger monitors as a standard part of their tech set up at home or at work.
To set up Responsive ad placeholders I would recommend to begin with just adding in one or two larger sizes to one or two placeholders.
As your site grows these placeholders will be generating data and you can then dig into this data to see if it is right for you to add these sizes to more placeholders.
When I am adding in the Responsive ad placeholders for Publishers I typically add in the 970 ad placeholders with a minimum screen size to ad placeholders 1 and / or 2.
You can do this by going into your Ezoic dashboard, at step 7 in this article https://support.ezoic.com/kb/article/guide-to-creating-placeholders-for-ezoic-ad-tester and then clicking on the select box for the sizes that you would like and adding in the relevant min. screen size and then click on save.