Displaying a widget only on long posts (over X characters)

Use the function t5_word_count() from this answer and extend the method widget() in your widget class:

public function widget( $args, $instance )
{
    if ( ! is_singular() )
        return;

    $content = get_the_content( '', TRUE );
    $words   = t5_word_count( $content );

    if ( 50 > $words )
        return;

    // print you widget
}