For green beans, pick them when they are about as big as a pencil. You want to get them when the little beans inside the pods are 1/4 inch or less in length. If you let them go longer, the pods lose their nice crisp texture. OTOH if you pick them too small, they don't have much body or flavor.
Have a great harvest!
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/- Plant a Garden
I've always "tested" my green beans by picking one single pod and eating it right there in the garden/growing area. If it tastes good, then I go ahead and gather the others of similar size and appearance, leaving the smaller ones to grow a little more.