Update the featured images size from Astra.

functions.php

<?php
/**
 * Update the featured images size from Astra
 */
add_filter( 'astra_post_thumbnail_default_size', 'update_featured_images_size_callback' );
 
function update_featured_images_size_callback( $size ) {
    $size = array( 500, 500 ); // Update the 500(Width), 500(Height) as per your requirment.
	return $size;
}

Add Following CSS to make the featured image align right with the content.
Please selcet the Blog Post Structure as this from Astra Customizer setting --

@media (min-width: 769px){
	article.ast-article-post.has-post-thumbnail .entry-content {
	    float: left;
	    clear: none;
	    width: 45%;
	}

	article.ast-article-post.has-post-thumbnail .ast-blog-featured-section.post-thumb {
	    float: right;
	    width: 50%;
	}
}