Potato/Multiplier onions are pretty small, compared to a regular onion you'd see in the grocery store. These are usually planted in the fall and pulled in June or July. Mine arrived yesterday and I'll plant in the next couple of weeks. You can read about them at
www.southernexposure.com and click on the link on their home page. There are shallots there too, which are similar. Most of these store very well.
From Italy, there are also cipollini onions, which are kind of flattened and make great pickling onions. There are also red ones called bottle onions. I think you can see these at
www.milkranch.com in the onion section. These Italian onions are sweet and I don't think they store very well but they are small. I think these are usually sold as seed but hopefully someone will speak up if I'm wrong about that.
Have fun!