Generate smaller post thumbnail for archives

I think from your question you might want to look at adding a new thumbnail image size into your functions.php

add_image_size( $name, $width, $height, $crop );

https://codex.wordpress.org/Function_Reference/add_image_size

You’ll then need to regenerate the post thumbnails, as you’ve added a new image size, there is a plugin called “Regenerate Thumbnails”

https://wordpress.org/plugins/regenerate-thumbnails/

Then you’ll need to set the front-page.php to use this new thumbnail image size when it constructs the blog index – themes do this in various ways, generally you’ll need to look at

the_post_thumbnail( $size, $attr );