How do I know when my snap beans are ready to pick

Pick. Less that 4", leave. If the beans go to maturity, the plant starts to shut down.

There is no exact size to pick as the size depends on the cultivar and the growing circumstances. The general rule is that the whole thing should be young and tender. This is while the seeds in the pod are underdeveloped and still small and soft. If they are big and bulgy or hard they are too old. If the pods are very thin with no visible seed they are probably too young. In this case they will still be tasty and tender but your yield will be down somewhat. If they are producing faster than you can eat them at full size you might choose to pick at this stage and enjoy the luxury of baby beans.

David