I grow them in VA, so not exactly the north.
Sweet potatoes require a long season, and most of the tuber growth occurs at the end. So, it's possible small tubers are all you can get there. But it's possible you can get bigger ones in the ground.
Yams are a very different plant, though confusingly, sweet potatoes are sometimes called yams. Any type of sweet potato can be grown the same way, though different varieties are somewhat adapted to different climates.
