Adding dimensions/units autocad 2013

Good day,

      I am writing today to ask about adding dimensions to dimensionless objects (see attached). The drawing is of a slice of bone with pores. The dimension/unit should be 4mm x 4mm. It is important that they remain a realisitic size. I was wondering how to assign this dimension to my drawing. Thank you for our time.

   kind regards,

          Rawan 

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