I would say dig them when you get enough that are the size you want. I know if you only have a few fines you don't want to go digging them all up before they get ready. Usually you can get some idea as to the way the ground would be swelled up around the plants. If frost is predicted you need to have the fines mowed off before it happens. You want to have them dug before ground temperature gets below I think 50 F but it might be 55 F. When you dig them do not wash them. It's been awhile since I had any and I need to review the curing process but this is what I remember. Store them in a area at about 85 degrees F and about that much humidity for two weeks. You should be able to find information from AR or MS Extension Service. Once they are cured they can be stored cooler and at lower humidity but don't let the temp go below 50 F or it could have been 55 F. Curing heals the skint places and broken ends. It will also make them sweeter better eating potatoes. Once you wash them they do not keep as long.