difference between size and length when using AR and DISTINCT

printer.rb

class Printer < ActiveRecord::Base
  belongs_to :manufacturer
  has_many :printer_cartridges
  has_many :cartridges, :through => :printer_cartridges

  attr_accessible :name, :id, :manufacturer_id
  attr_accessor :manufacturer, :group, :page, :pages, :per_page, :total

  def initialize(args = {})
    super
  end

  def printer_with_qraex_product
    printers = {}

    #self.total = select_printer.size
    self.total = select_printer.length

    select_printer(true).each do |p|
      cartridge_id = PrinterCartridge.select('cartridge_id').where('printer_id=?', p.id)[0].cartridge_id
      printers[p.name] = {qraexid: Cartridge.find(cartridge_id).qraexid}
    end
    printers
  end

  def pages
    (total.to_f / per_page.to_f).ceil
  end

  def self.id_from_name(printer_name)
    Printer.select('id').where('name=?', printer_name)[0].id
  end

  private

  def select_printer(pagination = false)
    if pagination
      Printer.select("DISTINCT printers.id, printers.name")
        .joins(:manufacturer, :printer_cartridges)
        .where(
          :manufacturers => {:shortname => manufacturer},
          :printer_cartridges => {:printer_group_id => group}
        ).order('printers.name').paginate(:page => page, :per_page => per_page)
    else
      Printer.select("DISTINCT printers.id, printers.name")
        .joins(:manufacturer, :printer_cartridges)
        .where(
          :manufacturers => {:shortname => manufacturer},
          :printer_cartridges => {:printer_group_id => group}
        ).order('printers.name')
    end
  end
end