You can do this by checking the order of the posts and applying the correct image size:
{{#if featuredPosts}}
<ol class="container">
{{#each featuredPosts}}
<li>
{{#featuredImage}}
{{#if url}}
<img
{{#checkIf @index '==' 0}}
src="{{urlLg}}"
{{else}}
src="{{urlXs}}"
{{/checkIf}}
{{#if @config.site.responsiveImages}}
{{responsiveImageAttributes 'featuredImage' srcset.listing sizes.listing}}
{{/if}}
{{ lazyload "lazy" }}
height="{{height}}"
width="{{width}}"
alt="{{alt}}">
{{/if}}
{{/featuredImage}}
</li>
{{/each}}
</ol>
{{/if}}
More about the image settings: https://getpublii.com/dev/how-images-work-in-publii/#advanced
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