A lot countries use what’s known as a screen size sorting system to classify their coffee beans. The main theory at play, is that if a bean is grown at a higher altitude, it is more likely to be larger (and more potent in flavor) than ones grown at a lower altitude.
Beans that are grown at higher altitudes are known to have a more potent flavour. These beans will grow more slowly at these higher altitudes, which gives it more time to develop these unique flavours. This is also why what we know as ‘premium’ blends often advertise that they are grown in the mountains.
To classify the coffee beans, and to group similarly sized beans together for a uniform roast, coffee producers pass the beans through a series of coffee screens on incrementally smaller sizes to sort these. Often the sizes are in 64’s of an inch, for example 18/64 beans would be larger than 13/64 beans. The larger the bean generally the better flavor but be careful to generalize, there are a lot of factors, especially the roasting that play into the quality of the coffee beans.