class Board
  attr_reader :width, :height

  def initialize(opts = {})
    @width  = opts[:width]  || 5
    @height = opts[:height] || 5
  end
end

class Board
  DEFAULT_WIDTH  = 5
  DEFAULT_HEIGHT = 5

  attr_reader :width, :height

  def initialize(opts = {})
    @width  = opts[:width]  || DEFAULT_WIDTH
    @height = opts[:height] || DEFAULT_HEIGHT
  end
end

Use constents to define default values
Use constents to define default values
