a.rb

require File.join(File.dirname(__FILE__), 'boot')

Rails::Initializer.run do |config|
  config.gem 'logging', :lib => false
  config.gem 'lockfile', :lib => false

# local libs/gems that can/must be loaded *inside* rails initializer
#
  require 'logging' unless defined?(Logging)
  require 'lockfile' unless defined?(Lockfile)

# force gem loading now
#
    Rails::Initializer.run(:add_gem_load_paths, config)
    Rails::Initializer.run(:load_gems, config)

  # do the actual logging configuration - this one will keep 7 log files of
  # roughly 42 mega-bytes on disk and fill not your lovely drive
  #
    require 'fileutils'
    FileUtils.mkdir_p File.join(RAILS_ROOT, 'log')

    logpath       = config.log_path
    number_rolled = 7
    megabytes     = 2 ** 20
    max_size      = 42 * megabytes
    options       = { :safe => true } # for multi-process safe rolling

    logger = ::Logging.logger(
      logpath,
      number_rolled,
      max_size,
      options
    )

    config.logger = logger
    config.logger.level = :info if RAILS_ENV=='production'
end

In Rails you can simply write 42.megabytes ;-)