Please go to Settings > Media, there you may change image sizes, for example, set Medium size – 600×600 and Large size – 1300×1300. Don’t forget Regenerate thumbnails https://wordpress.org/plugins/regenerate-thumbnails/
also, add this code in Theme Options > Styling > Custom css: article.blog-post .wp-picture img, article.post-grid .wp-picture img {width: auto;}
