What is the smallest monkey you have seen?

Question: What is the smallest monkey you have seen?
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Asked by to Lewis on 13 Mar 2014. This question was also asked by .
- Lewis Dean answered on 13 Mar 2014:
  
  I think the smallest that I have seen are marmosets. They are quite small (one would sit on your hand). They also live in nice little families, where the children hang around to help bring up their younger brothers and sisters.

Question: What is the smallest monkey you have seen?