Larger coffee beans are traditionally associated with higher quality. Because of this, some coffee-producing countries grade their beans by size. The largest beans, such as the Colombian Supremo or the Kenyan AA grades, are generally considered the most desirable and fetch the highest prices.
Scientific research does not entirely support the link between bean size and quality, however (Brando 2019). This should not be much of a surprise, considering that coffee beans from Ethiopia, which are generally very small, can offer extraordinary cup quality.
The size and shape of coffee beans depends on a multitude of factors, including the variety, growing conditions, and level of ripeness. Larger beans can be linked to factors that increase quality, such as sufficient water during fruit expansion and the availability of nutrients — but also to factors that decrease quality, such as lower elevations and warmer growing temperatures.
Smaller beans are more likely to harbour defects, however. All types of defective beans tend to be smaller than non-defective ones, for both arabica and robusta (Franca and Oliveira 2008. Sorting beans by size makes the subsequent separation of defective beans by density and in optical sorters more efficient (Brando 2019).
Peaberries are not clearly linked with better or worse quality than flat beans. Some varieties are genetically more predisposed to form peaberries than others — for instance, Rume Sudan produces about twice as many peaberries as Caturra or SL-28 (van der Vossen 1985).
Bean size and shape affect roasting in a number of ways, so careful screening of green coffee to ensure that the beans in a given lot are identical can help roasters obtain a more even result. Modern hot-air roasting machines, however, can successfully roast beans of different sizes together (Brando 2019).
The most important effect of bean size in roasting is simply that it takes longer for heat to penetrate a larger bean.