limit_post_body_size.rb

class LimitPostBodySize
  attr_accessor :app, :max_size
  def initialize(app, max_size); @app, @max_size = app, max_size; end
  def call(env)
    if env['rack.input'].stat.size < @max_size
      @app.call(env)
    else
      [413, {'Content-Type' => 'text/plain'}, 'Request Entity Too Large']
    end
  end
end

runner.ru

use LimitPostBodySize, 4096 # 4k is reasonable
run app