I tend to pick when they are 3 or 4 inches in length, they will get bigger but also tougher.

I make sure I pick before you can see the shape of a bean through the pod Lilaclady. Different bean varieties grow to different size lengths so that is not always the best way to judge I find, although most will still be nice and tender at 6 inches long x 1/2 in wide.

Yes, unless you are growing them for their beans to use in stews etc the pods as said above should be picked when young and tender.