I began this unit by reading Jack and the Beanstalk to my students. Then as with any book we read, the students answered a number of questions and retold the story. Next, I asked the students to tell me different characters and important things from the story. Each thing they told me, I wrote on a note card. The students then put three or four of the cards in order from lightest to heaviest, shortest to tallest, and so on.
Next, we created a chart of the characters and objects from the story, what we could measure on the characters, and what we would use to measure them. I used this to get an insight into what the students already knew and where I needed to go with my teaching.