They are the size of a small-sized regular potato. The dark, purple skin makes it harder to see them when digging them up from the ground.
Some purple colour will get on your fingers when peeling them, but it will wash off. The purple colour of the skin and flesh makes it harder to see bad spots that need to be cut out.
More of their colour will be retained if they are steamed rather than boiled.
Better for mashing rather than as a potato to serve boiled and whole.
Some purple potatoes may be acceptable for home canning quality-wise, others may not. It will depend on the actual variety. The National Center for Home Food Preservation says, ” From what we have read, there is a wide variety in the types and amounts of starches in blue potatoes, so not all blues are the same, just like not all white potatoes are the same in these characteristics.”  NCHFP blog posting. Preserving Potatoes. 6 October 2014. Accessed July 2016. .https://preservingfoodathome.com/2014/10/06/preserving-potatoes/
Foodies in the 1800s were interested in them for a while, but then the potatoes dropped off the radar screen. They even made it to Eastern Europe, though the varieties that made it there wouldn’t hold their colour when cooked. Now making a comeback (as of 2003).
|↑1||NCHFP blog posting. Preserving Potatoes. 6 October 2014. Accessed July 2016. .https://preservingfoodathome.com/2014/10/06/preserving-potatoes/|