What you can do is work with this `the_excerpt()` change this instead of `the_content();`

And give the amount of words that you want, add this in functions.php after you add the_excerpt();

    function custom_excerpt_length( $length ) {
        return 20;
    }
    add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

More info here: https://developer.wordpress.org/reference/functions/the_excerpt/

After that force the first line, using this for example. 

    .post p {
        width: 100px;
        white-space:nowrap;
        overflow:hidden;
    }

add the width of the content.(even if its responsive just change the css as you want).