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If I were assessing Paul's activity, I'd use

• notes about how the different groups and their members interacted with the material throughout the lesson (anecdotal evidence)
• checking for accurate mesurements (could everyone use a ruler and record in meters successfully?)
• check to see who was able to make the metric conversions
• listen to everyone as they shared their answers and explained their stories, what is their thinking?
• who made the connections from the mesurements we collected and the question "How big is water molecule?" 

These assessments would be largely to determine, what do I need to keep working on?  What is still confusing?  Who needs more help and experience with what?  Where is understanding still being built, still evolving?  Who needs more support with the concepts, who needs to build their skills, who needs enrichment?  I'd like to think more about asking for their understanding about comparing the relative sizes of the different things we measures (similar to Stephen's answer of how many water molecules fit across a hair...)

My eventual goal , and what I think would be an authentic assessment, would be what Stephan was describing: applying and using scale in other projects.  I would want to engage the students in more experiences and practice with scale before using this kind of assessment.