Well it depends on whether you are trying to heat a leaky pup temp at the North Pole or if you have a supper insulate house in Florida.

You need to do a heat loss calcualtion.

Check the HVAC forum.

As a rough rule of thumb in the area I live in (WV), 10 watts per square foot in a reasonably well insulated house will maintain a 90 degree temperature differential (-20 outside, + 70 inside).

But to know for sure, the heat loss calculation suggested by Bill is a must. They aren't that hard to do, but you do need to have quite a few measurements on hand.