I solved it myself 
Here is my solution for anyone trying to achieve the same thing:
In product-block.liquid I added style=“{% if section.settings.smaller_tiles %} max-width: 260px;{% endif %}” to the <div class="product-block
In featured-collection.liquid I added the following code to the schema-tag: {
“type”: “checkbox”,
“id”: “smaller_tiles”,
“label”: “Display smaller product tiles”,
“default”: false
}
This allows to check/uncheck an option for smaller tiles in the frontend editor of your featured collection section.