I pick them at various stages. The really young ones i like to steam or stir fry. The medium ones i like to cut in soups. The bigger ones, I like to cut in 1/4 inch thich the long way , bread and fry them up and make a Zuc parmesan hero, with sauce, cheese ... they fit perfectly on an italian bread when u cut them that way. The huge ones, are tough and more bitter and seedy.
OOPS!!! we're talking about beans
for me, as long as u cant see the individual 'beans' swelling through the outer pod, I consider them ok. Once the inside bean gets too big, the outer part gets fibrous and chewy, and the bean is more starchy. So, for me, better too soon then too late. When i get an excess i pick, cut, blanch and freeze for later use in soups, but much better raw or freshly cooked.