If you have to deal with 'zero degree nights' and MUST run the furnace, then get some advice from a qualified electrician on the startup, peak and continuous load your heating systems creates. With this knowledge in hand, there are fairly simple worksheets available fomr the generator manufacturers to calcualte the capacity generator you need.
Don't guess at it. Get the math right.
In NC mountains we're ususally dealing with temps in the teens at night in winter. AND we have the wood stove, so we DON'T need to run the furnace.
Getting the generator size established is FREE help you can get from the retailers. Do it ;-).
Another thought I had, was to get a furnace that burns both oil, and something else (since our furnace is old, we're kind of due), if there is such a thing, and a small generator for frige, lights and water pump.
Our fireplace is OK for the first day, but with each day, the house gets colder. An old-fashioned hot water bottle really helps (but not good for pregnant women, btw).