How do I increase the upload size only when editing special pages?

I think you are on the right track with your idea of checking if the right page is being viewed within the callback attached to the upload_size_limit hook.

This code demonstrates changing the upload size if one of the special pages is being viewed, and returning the standard max upload size otherwise:

function wpse239631_change_upload_size( $u_bytes, $p_bytes ) {
    // Array of post IDs where the upload size will be changed.
    $special_pages = array(
        1829, // change to your page
        1800, // change to your page, etc
    );

    // If we're on a special page, change the upload size,
    // otherwise, use the default max upload size.
    if ( in_array( get_the_ID(), $special_pages ) ) {
        return 1024 * 18000;
    } else {
        return min( $u_bytes, $p_bytes );
    }
}
add_filter( 'upload_size_limit', 'wpse239631_change_upload_size', 10, 2 );