The physical properties such as the length, width, thickness, volume, density and moisture contents of the beans differentiate coffee beans in to defective (immature, sour and black) and   non-defective beans at raw and roasted  level

Thank you Dr. Belay for your answer. Considering the non-defective beans, what are the most relevant quality parameters that you would mention, for a classification into "high quality" or "superior quality"?

size of coffee bean

Thank you Leonel: apparently it should be quite easy to detect physical defects, but what about chemical properties? It cup tasting the only way of detecting defects (or low quality)?

I think you can check the clorogenic acid content

All submissions are important. But you should never discard an important physical parameter - bean size. Before coffee beans are exported from originating countries they are sorted into size grades which is the primary parameter for pricing the coffee beans among other factors. Given that all the desirable chemical traits are present, the coffee roasting industry would not still appreciate a lot of mixed sizes as bigger coffee beans roast more slowly than smaller beans, hence the result is inconsistent cup quality. Therefore, uniformity of the coffee lot is key.