I am in search of a snippet or function that will filter the Embedded images inside a post to  a custom size I have defined in functions.php. 

To make Clear Here is the scenario - In a blog Client does not care about the image sizes he insert inside the post using add media button, so some times he will upload very large images and if i use CSS to resize image it will not always work correctly. So I think If is there a way to get the embedded images in a custom size that I have already defined to fit the layout dynamically using Filter.

Using the below code I get all the attachments i have inside the post, and can loop through it,

        $attachments = get_posts($args);
    if ($attachments) {
        foreach ($attachments as $attachment) {
            .....
        }
    }

Now, Is it possible to display custom sized images in posts, even if uploaded Larger images inside editor. 


EDIT:

summarize as  - Search the content for any attachments coming through the_content(), and then display the Custom size versions of those images(those we defined inside functions.php) in front end rather showing the original image uploaded.

TO MAKE IT MORE SPECIFIC HERE IS A SAMPLE CODE, I am not a PHP geek, I know it is a crap, but just a hint to what i want,

  

      function myCustomSize(){
            global $post;
            $args = array(
                                'post_type' => 'attachment',
                                'numberposts' => null,
                                'post_status' => null,
                                'post_parent' => $post->ID,
                                'exclude'     => get_post_thumbnail_id()
                            );
                            $attachments = get_posts($args);
                            if ($attachments) {
                                foreach ($attachments as $attachment) {
                                  return wp_get_attachment_image( $attachment->ID, 'themerpro_post_thumb' );
            
                                }
                            }
            
            }
            add_filter('the_content', 'myCustomSize');

What it do is it will return specified image(ie, themerpro_post_thumb), that is exactly I want, but the issue with the above code is, it will display only one image from the post, function returns an array, want to solve that, second issue is it will skip out the text inside the post(i mean paragraphs, etc...).