some_spec.rb

shared_examples_for "a measurable object" do |measurement_method, measurement|
  it "should return #{measurement} from ##{measurement_method}" do
    subject.send(measurement_method).should == measurement
  end
end

describe Array do
  it_should_behave_like "a measurable object", :size, 2 do
    subject { [2, 13] }
  end
end

describe String do
  it_should_behave_like "a measurable object", :length, 6 do
    subject { "FooBar" }
  end
end