def resize(rra_num, options)
      info = self.info
      step = info["step"]
      rra_step = info["rra[#{rra_num}].pdp_per_row"]
      action = options.keys.first.to_s.upcase
      delta = (options.values.first / (step * rra_step)).to_i # Force an integer
      Wrapper.resize(rrd_file, rra_num.to_s, action, delta.to_s)
    end
def resize(rra_num, options)
      info = self.info
      step = info["step"]
      rra_step = info["rra[#{rra_num}].pdp_per_row"]
      action = options.keys.first.to_s.upcase
      delta = (options.values.first / (step * rra_step)).to_i # Force an integer
      Wrapper.resize(rrd_file, rra_num.to_s, action, delta.to_s)
    end
Writes a new file 'resize.rrd'

 You will need to know the RRA number, starting from 0:

  rrd.resize(0, :grow => 10.days)