Creating custom link that redirects to a another link

First, create a page with the intended URL slug and note down the ID of it. Then in your functions.php

function wpse410694_custom_redirect() { 
    //Let us assume the ID of the page is 123
    if ( is_page(123) ) {
        wp_redirect( '' );
add_action( 'template_redirect', 'wpse410694_custom_redirect');

The maximum upload size is normally set from server. You can increase it by tweaking the server settings.