I have a package of tri color gourmet potatoes,red, purple & yellow, that I have let sprout, and plan to plant them. They are about the size of a walnut. Do they just grow to that small size, or are they just harvested when they become that size? They have sprouted very well.
I had a reply typed, then my ATT useless connection died, so here goes again.
When we dig potatoes, there are always several large ones, some medium ones and quite a few small, golfball and smaller size which we separate out and package separately. Some varieties are smaller normally, some commercial growers either spray something, or cut the tops early to harvest small potatoes. I think the variety Rio Colorado was designed for harvesting this way.
If you have "regular" potatoes, the All Blue and Purple Majesty varieties are blue inside and out with Purple Majesty being the normally smaller of the two. Cranberry and Mountain Rose are red outside and pink inside, with Mountain Rose being the smaller variety.
In fingerlings, Peruvian Purple is the only purple one I know of. It is a long season potato, taking about 120 days to mature, is dark blue inside and out with very dense, crisp flesh that is excellent roasted. There are several red outside, pink inside fingerlings. Blossom and Red Thumb are two that come to mind with Red Thumb being the smaller variety. These take 70-90 days and are both excellent as far as easy to grow, good yield and flavor.