I'm not really the best person to answer this (but since you've gotten no responses yet). I used to cut off the roots of store bought onions and plant them. The results never got as large as the original onion.

But I've done a little research, and picked up a few hints. In Zone 5 I'm guessing you need a long-day onion variety, or an intermediate-day variety. Work the soil well. Plant early. Keep out competition, so keep them weeded and keep the onions well spaced. They are heavy feeders, so fertilize regularly. Water well during the hot months.

There is an allium forum on GardenWeb for onions, garlic, leeks, etc.

Here is a link that might be useful: Allium Forum

Thank you for the link and the help, I will look into it. :)