prawn：Prawn::Box#bounding_box

#!/usr/bin/env ruby

require "prawn"
require "prawn/layout"

pdf = Prawn::Document.new
pdf.define_grid(:columns => 3, :rows => 2, :gutter => 10)

b = Prawn::Document::Box.new(pdf, 2, 3)
b.bounding_box do
  pdf.text "foo"
end

pdf.render_file(File.basename($0, ".rb") + ".pdf")