How to get override the WP Admin Syndication feed limit for a custom feed?

This is no normal filter. It’s a filter_ref_array.

// Use this to alter your limit:
add_filter( 'post_limits', create_function( '$a', "return 'LIMIT $start, 10';" ) );

EDIT

After I saw that this got marked as solution, I want to show you a more “best practice”-approach:

function alter_feed_post_limits( $limits )
{
    return " LIMIT 0,50";
}
add_action( 'post_limits', 'alter_feed_post_limits' );