title_body: What factors determine coffee bean size?
upvoted_answer: The single biggest factor in the size of a coffee bean is the botanical variety of the plant. There is no amount of nutrition or growing conditions that can overcome the effect of genetics.  
  
  In this picture from our garden at Coffea Diversa you can see two examples at the extreme. 
  
  
  On one hand you have mokka, this variety has the smallest bean of all
  coffee varieties. 
  On the other hand you have Maragogype, this variety has the largest
  bean of all coffee varieties.
  
  
  There is no amount of fertilizer that you can give to a mokka grain to make it even come close to the size of a ripe Maragogype bean (and viceversa). 
  
  The difference in sizes between your blend and peruvian coffees are probably due to the botanical varieties of the beans.