What factors determine coffee bean size? I bought a bag of Guatemala/Colombia/Ethiopia blend after having single origin Peru beans (all Arabica), and was struck by the difference in bean size: Peruvian beans were almost twice larger than the beans in the blend.

Now I'm wondering whether the difference is conditioned by some environmental factors, or regional differences of the coffee plant, or the timing of harvesting. Of course, it can also be that any coffee plant anywhere produces beans of (significantly) different size, and then they are sorted into size-consistent batches (as per this guide, for instance).
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.