Automatically use resized images instead of originals in WordPress aspect ratio?

I would appriciate Your help. I used custom field in WordPress and created the form to upload the image in the post. Everything works fine. I also placed this code to replace the original image ( if someone posted huge image size ) which automatically resizes it. It in fact resizes the image but it doesn’t keep the aspect ratio meaning max 500px width, max 800px height. It takes that walues and becomes croped to that size. I want the height to be proportional not croped! this goes to functions.php

function replace_uploaded_image($image_data) {
// if there is no large image : return
if (!isset($image_data['sizes']['large'])) return $image_data;

// paths to the uploaded image and the large image
$upload_dir = wp_upload_dir();
$uploaded_image_location = $upload_dir['basedir'] . '/' .$image_data['file'];
$large_image_location = $upload_dir['path'] . '/'.$image_data['sizes']['large']  
['file'];

// delete the uploaded image
unlink($uploaded_image_location);

// rename the large image
rename($large_image_location,$uploaded_image_location);

// update image metadata and return them
$image_data['width'] = $image_data['sizes']['large']['width'];
$image_data['height'] = $image_data['sizes']['large']['height'];
unset($image_data['sizes']['large']);

return $image_data;
}
add_filter('wp_generate_attachment_metadata','replace_uploaded_image');

http://pastie.org/1928349