To have a conceptual understanding of what measurement is, students must create their own unit of measurement. This lesson is meant to be done groups up to 7. It comes with 7 different tasks, which allows there to be 7 different groups. There are easier tasks and harder task which will help you differentiate your students by ability.
Example: They could use one dry erase marker as their one unit. They would then go measure the door with their dry erase markers. Thinking mathematically, they will then need to convert their units to inches or centimeters (or whatever you measurement you wish them to use). Once they have that, they will need to go measure their items again using rulers to see if there was a difference. (Students will discover that the more abstract their objects are the more errors will be in their results). This leads to a great discussion about why that is the case, and why measurement is used. Why it is important to have a secular measurement.