Some varieties are available in two sizes.  Small tubers are about the size of marbles and will each grow one plant.  Medium/large tubers are usually big enough to cut into at least two or three seed pieces.

Some potatoes are available for shipping now if you want to get an early start next year.  You will need to refrigerate them to keep them in good condition until spring.  I recommend storing them in a plastic bag with a few holes poked in it.