Hi There! Smaller beans are preferred, as the fermentation process will be able to reach the center of the bean more easily. I’m looking for small natto soybeans for myself too. Natto not just taste good with rice but really healthy food. Nattō is an extremely rich source of the MK7 variant of vitamin K2, with one study finding mean concentrations of 998μg MK7 per 100g of natto, over 500 times greater concentration than any other food tested.