specify size of post preview image (not post thumbnail)

In this case I would add image size in functions.php which registers new image size in my theme and get the re-sized image.

Example:

Register custom image size (functions.php)

if ( function_exists( 'add_image_size' ) ) { 
    add_image_size( 'custom-image', 440, 265, true ); //(cropped)
}

Get custom image size (functions.php)

function get_custom_image(){
    global $post, $posts;
    $args = array(
        'post_type' => 'attachment',
        'numberposts' => 1,
        'post_status' => null,
        'post_parent' => $post->ID
    );

  $attachments = get_posts( $args );
     if ( $attachments ) {
        foreach ( $attachments as $attachment ) {          
           $found =  wp_get_attachment_image( $attachment->ID, 'custom-image' );
           preg_match( '@src="https://wordpress.stackexchange.com/questions/99987/([^"]+)"@' , $found , $match );
           $img = $match[1];
           return $img;
          }
     }else{
         echo 'image_not_available_path_.jpg';
     }
}

Using inside loop

<div class="post-image">
        <img src="https://wordpress.stackexchange.com/questions/99987/<?PHP get_custom_image(); ?>" alt="cait barker post image" />
    </div><!-- end blog-image -->

Code not fully tested, you have to modify it a bit for your needs.