Really really cool idea!
However, I don't really understand how the comparison works. Maybe there could be a title like "How much bigger is a [cucumber]?" or something like that. Because sadly, I've lost every single estimation :(
Now the positive:
+ very very unique and innovative
+ very polished (transitions, music, etc
+ I love the measurement UI (I just love little things like that :) )
=> 4*, but I would absolutely give 5* if the game taught me what I "really" need to compare :)