Hi Jackie,
Potatoes are perfect for digging up at any stage of growth once the full foliage and flowers have formed. If you want to pick your tubers when they are at fingerling stage then that is the time to pick...Ideally for fully grown potatoes (such as russets) you would wait until the foliage has died back completely to dig them up. I made the mistake of picking my French fingerling potatoes (literally this week) once the foliage had died back and I had two pound fingerling potatoes!! ooops! Best!~ Ian