size.rb

class Size
  SIZES = %w(small medium big)
  attr_reader :size

  def initialize(size)
    @size = size
  end

  def self.to_select
    SIZES.map{|c| [c.capitalize, c]}
  end

  def valid?
    SIZES.include?(size)
  end

  def to_s
    size.capitalize
  end
end